Assessing interrill erosion rate from soil aggregate instability index, rainfall intensity and slope angle on cultivated soils in central Greece

In order to develop a new formula for assessing interrill erosion rate by incorporating the soil aggregate instability index, β, erosion plots at seven sites in central Greece were used to measure interrill erosion rate under natural rainfall conditions during a 39-month period. Soils classified as Alfisols, Inceptisols and Entisols with slopes 7–21%, moderately well to excessively drained, clay to loamy textured, were studied. Runoff and total sediment were collected after each ponding rainfall event. The equation E_i=0.628 β S_t^1.3 e^{0.0967I₃₀} was finally proposed (R²=0.939,P<0.001) to describe interrill erosion rate. The term, S_t represents the tangent of the slope angle, and I₃₀ represents the maximum rainfall intensity in 30 min. The addition of the aggregate instability index to improve existing methodologies provide was considered to provide an easy to determine and reliable measure of soil erodibility. Validation with independent data showed that the model predicted interrill erosion well (R²=0.766, P<0.001). Therefore, the proposed model based on the aggregate instability index, β, has the potential to improved methodology for assessing interrill erosion rate.     

Impacts of soil tilth on the effectiveness of biological geotextiles in reducing runoff and interrill erosion

The effectiveness of a surface cover material (e.g. geotextiles, rock fragments, mulches, vegetation) in reducing runoff and soil erosion rates is often only assessed by the fraction of the soil surface covered. However, there are indications that soil structure has important effects on the runoff and erosion-reducing effectiveness of the cover materials. This study investigates the impact of soil pre-treatment (i.e. fine tilth versus sealed soil surface) on the effectiveness of biological geotextiles in increasing infiltration rates and in reducing runoff and interrill erosion rates on a medium and steep slope gradient. Rainfall was simulated during 60 min with an intensity of 67 mm h⁻¹ on an interrill erosion plot having two slope gradients (i.e. 15 and 45%) and filled with an erodible sandy loam. Five biological and three simulated geotextiles with different cover percentage were tested on two simulated initial soil conditions (i.e. fine tilth and sealed soil surface). Final infiltration rates on a sealed soil surface (7.5–18.5 mm h⁻¹) are observed after ca. 10 min of rainfall compared to ca. 50 min of rainfall on an initial seedbed (16.4–56.7 mm h⁻¹). On the two tested slope gradients, significantly (α = 0.05) smaller runoff coefficients (RC) are observed on an initial seedbed (8.2% < RC < 59.8%) compared to a sealed soil surface (75.7% < RC < 87.0%). On an initial seedbed, decreasing RC are observed with an increasing simulated geotextile cover. However, on an initial sealed soil surface no significant effect of simulated geotextile cover on RC is observed. On a 15% slope gradient, calculated b-values from the mulch factor equation equalled 0.054 for an initial fine tilth and 0.022 for a sealed soil surface, indicating a higher effectiveness of geotextiles in reducing interrill erosion on a fine tilth compared to a sealed soil surface. Therefore, this study demonstrates the importance of applying geotextiles on the soil surface before the surface tilth is sealed due to rainfall. The effect of soil structure on the effectiveness of a surface cover in reducing runoff and interrill erosion rates, as indicated by the results of this study, needs to be incorporated in soil erosion prediction models.     

Interrill erosion from disturbed and undisturbed samples in relation to topsoil aggregate stability in red soils from subtropical China

The assessment of soil erodibility to water erosion in the field is often expensive and time-consuming. This study was designed to reveal the effects of aggregate breakdown mechanisms on interrill erosion dynamics and develop an improved model for assessing interrill soil loss, which incorporated the soil aggregate stability tests as a substitute for the interrill erodibility parameter, from both disturbed and undisturbed samples for red soils in subtropical China. Six cultivated areas of sloping land with red soils were selected, and topsoil aggregate stability was analyzed using the Le Bissonnais method to determine the different disaggregation forces. Laboratory rainfall simulations were designed to distinguish the effects of slaking (at different wetting rates) and mechanical breakdown (with and without screening) on soil erosion characteristics. Field rainstorm simulations with medium and high rainfall intensities were conducted on runoff plots (2 m 1 m) with slope gradients varying from 10% to 20% for each soil type. A new instability index, Ka, which considers aggregate breakdown mechanisms in interrill erosion processes, was proposed based on the disturbed sample results. Ka showed a close relationship with erosion rates in both disturbed and undisturbed samples. Following from the results of undisturbed sample experiments, Ka was used as a substitute for the erodibility factor, and introduced into the WEPP model, establishing a new erosion predication formula for red soils which had a good correlation coefficient (R² = 0.89**). This research made a good attempt at estimating the interrill erosion rate on the basis of aggregate stability from simple laboratory determinations. These results extend the validity of soil aggregation characterization as an appropriate indicator of soil susceptibility to interrill erosion in red soils from subtropical China. The formula based on the instability index, Ka, has the potential to improve the methodology used for assessing interrill erosion rates.     

Evaluation of ultrasonic aggregate stability and rainfall erosion resistance of disturbed and amended soils in the Lake Tahoe Basin, USA

Application of organic soil amendments to disturbed soil has been shown to improve aggregate stability and reduce soil susceptibility to erosion. Employing ultrasonic aggregate stability assessment techniques described earlier [Fristensky, A. and Grismer, M.E., 2008. A simultaneous model for ultrasonic aggregate stability assessment. Catena, 74: 153–164.], we assess the effect of two experimental organic soil amendments – a compost and a woodchip mulch incorporated at a rate of 2000–6000 kg ha^− 1 N-equivalence – on soil aggregation and aggregate stability at four drastically disturbed sites within the Lake Tahoe Basin, USA. Experimental plots were established 1–3 years prior to testing. The soils were of granitic or volcanic origin, and disturbed by either ski run or road development. Soil treatments were observed to significantly (p < 0.05) increase both aggregation (300% average increase) and ultrasonic aggregate stability (600% average increase) relative to the untreated soil. However, at the two sites disturbed by ski run development, the control treatment (tilling and surface application of pine–needle mulch) performed comparably to the two incorporated compost treatments, suggesting that the effects of the experimental amendments on aggregation were negligible at these sites, or their effective duration was shorter than the evaluation period.Rainfall simulations (72–120 mm h^− 1) were performed on the treatment plots, and results were compared with the ultrasonic aggregate stability indices. Significant (p < 0.05) positive correlations were obtained between the measurements of aggregate instability and indices of soil susceptibility to runoff, including steady-state infiltration rate (measured values between 1 and 120 mm h^− 1), and the level of kinetic energy of applied rainfall at which runoff commences (EBR, measured values between 12 and 224 J m^− 2). However, no correlation was found between the ultrasonic aggregate stability indices and observed soil erosion variables. Interestingly, positive relationships (p < 0.05) were observed between both infiltration rate and EBR and the proportion of 2–20 μm and < 2 μm particles liberated from the largest aggregates detected in each soil. Our results suggest that ultrasonic aggregate stability indices may be useful indicators of soil susceptibility to runoff and erosion under rainfall.     

Effects of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing

Soil erosion during rainfall is strongly affected by runoff and slope steepness. Runoff production is drastically increased when a seal is formed at the soil surface during rainfall. Therefore, a complex interaction exists between soil erosion and surface sealing. In this study, the dynamics of interrill erosion during seal formation is studied under different simulated rainfall and slope conditions. A sandy soil was exposed to 70 mm of rainfall at two intensities, 24 mm h^− 1 and 60 mm h^− 1, and five slope gradients, from 5% to 25%. Infiltration, runoff and soil loss rates were monitored during rainfall. Final infiltration rates increased with slope gradient at both rainfall intensities, this effect being stronger for the higher intensity. Cumulative runoff at the end of the rainfall event was lower as slopes were steeper, while an opposite trend was obtained for soil loss. For the 5% and the 9% slopes, the sediment concentration in runoff reached quickly a stable value during the whole rainfall event, while it reached a peak value before declining for the higher slopes. The peak value and its timing were rainfall intensity dependent. Soil erodibility during seal formation was evaluated using two empirical multiplication-of-factors type models. It seems that slope and rainfall erosivity are accounted for only partly in these models. For mild slope gradients below 9%, the value of K_i estimated by means of the two expressions becomes practically constant shortly after runoff apparition. Consequently, the estimates resulting from this type of expressions remain valuable from the practical point of view.     

A method for estimating coefficients of soil organic matter dynamics based on long-term experiments

The one-compartment C model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t) is being long used to simulate soil organic C (SOC) stocks. C_t is the SOC stock at the time t; C₀, the initial SOC stock; k₂, the annual rate of SOC loss (mainly mineralization and erosion); k₁, the annual rate to which the added C is incorporated into SOC; and A, the annual C addition. The component C₀e^−k₂t expresses the decay of C₀ and, for a time t, corresponds to the remains of C₀ (C_0 remains). The component k₁A/k₂(1−e^−k₂t) refers, at time t, to the stock of SOC derived from C crops (C_crop). We herein propose a simple method to estimate k₁ and k₂ coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions (A) and SOC stocks. We estimated k₁ and k₂ for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N-urea rates (0 kg N ha⁻¹ −0 N and 180 kg N ha⁻¹ −180 N) in a long-term experiment established in a subtropical Acrisol with C₀ = 32.55 Mg C ha⁻¹ in the 0–17.5 cm layer. A linear equation (C_t = a + bA) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t), so that a=C₀e^−k₂t and bA=k₁A/k₂(1−e^−k₂t). Such equivalences thus allow the calculation of k₁ and k₂. NT soil had a lower rate of C loss (k₂ = 0.019 year⁻¹) than CT soil (k₂ = 0.040 year⁻¹), while k₁ was not affected by tillage (0.148 year⁻¹ under CT and 0.146 year⁻¹ under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state (C_e) in the 0–17.5 cm layer ranged from 15.65 Mg ha⁻¹ in CT O/M 0 N to 60.17 Mg ha⁻¹ in NT OV/MC 180 N. The SOC half-life (t_1/2 = ln 2/k₂) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C_0 remais), while increments in C_crop are imparted mainly by crop additions.     

Environmental control of net ecosystem CO2 exchange in a treed, moderately rich fen in northern Alberta

Peatlands cover about 21% of the landscape and contain about 80% of the soil carbon stock in western Canada. However, the current rates of carbon accumulation and the environmental controls on ecosystem photosynthesis and respiration in peatland ecosystems are poorly understood. As part of Fluxnet-Canada, we continuously measured net ecosystem carbon dioxide exchange (NEE) using the eddy covariance technique in a treed fen dominated by stunted Picea mariana and Larix laricina trees during August 2003–December 2004. The total carbon stock in the ecosystem was approximately 51,000 g C m⁻², with only 540 g C m⁻² contributed by live above ground vegetation. The NEE measurements were used to parameterize simple physiological models to assess temporal variation in maximum ecosystem photosynthesis (A_max) and ecosystem respiration rate at 10 °C (R₁₀). During mid-summer the ecosystem had a relatively high A_max (approx. 30 μmol m⁻² s⁻¹) with relatively low R₁₀ (approx. 4 μmol m⁻² s⁻¹). The peak mid-day NEE uptake rate during July and August was 10 μmol m⁻² s⁻¹. The ecosystem showed large seasonal variation in photosynthetic and respiratory activity that was correlated with shifts in temperature, with both spring increases and fall decreases in A_max well predicted by the mean daily air temperature averaged over the preceding 21 days. Leaf-level gas exchange and spectral reflectance measurements also suggested that seasonal changes in photosynthetic activity were primarily controlled by shifts in temperature. Ecosystem respiration was strongly correlated with changes in ecosystem photosynthesis during the growing season, suggesting important links between plant activity and mycorrhizae and microbial activity in the shallow layers of the peat. Only very low rates of respiration were observed during the winter months. During 2004, the peatland recorded a net annual gain of 144 g C m⁻² year⁻¹, the result of a difference between gross photosynthesis of 713 and total ecosystem respiration of 569 g C m⁻² year⁻¹.     

Organic carbon stocks and soil erodibility in Canary Islands Andosols

Soil organic carbon (SOC) plays a key role in the structural stability of soils and in their resistance against erosion. However, and as far as andic soils are concerned, these mechanisms and processes, as well as the influence of the different types of SOC on aggregate stability, are not fully understood. The targets of this paper are: (i) to determine the content and forms of SOC in Andosols under evergreen forest vegetation [laurel (Laurus) and heather (Erica) forest] and (ii) to find out the role of soil organic matter (SOM) in the aggregate stability and in the resistance of Andosols to water erosion. Soil samples have been collected in 80 sites in a 40 km² area under udic soil moisture regime. In them, fulvic and humic acids, Walkley–Black SOC, pyrophosphate-extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC, acid oxalate-extractable Fe, Al and Si, USLE K-factor and aggregate stability have been determined. The Andosols over volcanic ash are Aluandic Andosols (non-allophanic Andosols), whereas over basaltic lava flows are Silandic Andosols (allophanic Andosols). The surface (0–30 cm) samples analyzed contain 9.5–30 kg C m^− 2 being significantly higher in allophanic Andosols (p < 0.5). Organic carbon adsorbed onto the mineral fraction (extractable pyrophosphate, C_p) accounts for 35–55% of the total SOC. All samples show a high stability to slaking and raindrop impact, being the first one highly correlated (r = 0.6) with pyrophosphate extractable C (C_p), Fe (Fe_p), and Al (Al_p) in allophanic Andosols, unlike non-allophanic ones. The stability to raindrop impact correlates with pyrophosphate extractable C (C_p) and Fe (Fe_p) in both types of soils (r = 0.3–0.6, p < 0.05). These findings suggest that the high stability to both slaking and water-drop impact is due to the occurrence of allophane–Fe–OC complexes, rather than to the total OC, and the active Fe and Al forms, generated by the weathering of volcanic materials, constitute an essential constituent responsible for C sequestration and resistance to degradation in these soils.     

Carbon and nitrogen distribution in water-stable aggregates under two tillage techniques in Fluvisols of Owerri area, southeastern Nigeria

Organic matter influences soil structure and compactibility by binding soil mineral particles, reducing aggregate wettability, and influencing the mechanical strength of soil aggregates, which is the measure of coherence of inter-particle bonds. This work was carried out to examine how differences in water-stable aggregates influence the distribution of soil organic carbon and soil organic nitrogen under two tillage techniques [minimum tillage (only planting holes were opened) and conventional tillage (raised beds, 30 cm high, prepared manually with traditional hoes)] in soils of a Fluvisol in Owerri, southeastern Nigeria. Three pedons were dug and studied for each of the tillage technique along a soil sequence. Soil organic carbon and soil organic nitrogen distribution in whole soil and in water-stable aggregates under minimum tillage and conventional tillage were determined for the soils. Soil organic carbon contents in water-stable aggregates (WSA) of the pedons varied according to method of tillage. The highest mean values of soil organic carbon were obtained from minimum tillage and in water-stable aggregates 4.75–2.00 mm (16.03 Mg C ha⁻¹), 1.00–0.50 mm (14.06 Mg C ha⁻¹) and water-stable aggregates 2.00–1.00 mm (13.99 Mg C ha⁻¹) whereas under conventional tillage, water-stable aggregates 1.00–0.50 mm with soil organic carbon of 24.6 Mg C ha⁻¹ had the highest soil organic carbon content. Soil organic carbon correlated significantly with mean weight diameter (r = 0.48; P = 0.05; n = 15), water-stable aggregates 4.75–2.00 mm (r = 0.73; P = 0.05; n = 15), water-stable aggregates 2.00–1.00 mm (r = 0.55; P = 0.05, n = 15), water-stable aggregates 1.00–0.50 mm (r = 0.44; P = 0.05; n = 15) whereas no relationship was found between soil organic carbon and water-stable aggregates 0.50–0.25 mm (r = 0.15; P = 0.05; n = 15) and water-stable aggregates <0.25 mm (r = 0.17; P = 0.05; n = 15) in soils under minimum tillage. There was a significant correlation (r = 0.45–0.58; P = 0.05; n = 14) between all water-stable aggregates classes studied and soil organic carbon in soils under conventional tillage. Mean values of soil organic nitrogen were higher in soils under minimum tillage with 4.75–2.00 mm and 2.00–1.00 mm aggregate classes having 1.64 Mg N ha⁻¹ and 1.57 Mg N ha⁻¹ soil organic nitrogen when compared to 1.01 Mg N ha⁻¹ and 1.00 Mg N ha⁻¹ in conventionally tilled soils of the same aggregate classes, respectively. Larger water-stable aggregate classes (4.75–2.00; 2.00–1.00) had slightly more soil organic nitrogen (22–26%) than smaller aggregate classes (1.00–0.50; 0.50–0.25; >0.25) with 14–24% soil organic nitrogen in minimum tilled soils. In soils under conventional tillage, 1.00–0.50 mm, 0.50–0.25 mm and <0.25 mm aggregate classes contributed more soil organic nitrogen (19.66–22.40%) to the soil whereas larger water-stable aggregate classes contributed 19.22% soil organic nitrogen. The proportion of soil organic carbon and total nitrogen retained in soils with higher percentage of water-stable aggregates are less likely to be lost through soil and wind erosion. The higher values of SOC in the whole soil and WSA classes less than 2.00 mm are indications of positive influence of SOC on the stability of these peds.     

Improved legume tree fallows and tillage effects on structural stability and infiltration rates of a kaolinitic sandy soil from central Zimbabwe

Improved legume tree fallows have great potential to increase soil organic carbon (SOC), aggregate stability and soil infiltration rates during the fallowing phase. However, persistence of the residual effects of improved fallowing on SOC, aggregate stability and infiltration rates, under different tillage systems in Zimbabwe is not well documented. The relationships between SOC, aggregate stability and infiltration in fallow-maize rotation systems are also not well documented. We therefore evaluated effects of tillage on SOC, aggregate stability and infiltration rates of a kaolinitic sandy soil during the cropping phase of an improved fallow-maize rotation system. Plots that were under legume tree fallows (Sesbania sesban; Acacia angustissima), natural fallow (NF) and under continuous maize during the previous 2 years were divided into conventional tillage (CT) and no-till (NT) subplots soon after fallow termination, and maize was cropped in all plots during the following two seasons. Aggregate stability was investigated using water stable macroaggregation index (I_ma), water dispersible clay (WDC) and using the mean weight diameter (MWD) after different wetting procedures. Infiltration rates were determined using simulated rainfall at intensity of 35 mm h⁻¹ on 1 m² plots. Soil organic carbon was significantly higher (P < 0.05) under fallows than continuous maize. For the 0–5 cm depth SOC was 11.0, 10.0, 9.4 and 6.6 g kg⁻¹ for A. angustissima, S. sesban, NF and continuous maize, respectively, at fallow termination. After 2 years of cropping SOC was 8.0, 7.0, 6.1 and 5.9 g kg⁻¹ under CT and 9.1, 9.0, 8.0 and 6.0 g kg⁻¹ under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. Aggregate stability was significantly greater (P < 0.05) under fallows than under continuous maize and also higher under NT than under CT. The macroaggregation index (I_ma) for the 0–5 cm depth was 466, 416, 515 and 301 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination, decreasing to 385, 274, 286 and 255 under CT and 438, 300, 325 and 270 under NT, for A. angustissima, S. sesban, NF and continuous maize, respectively, after 2 years of cropping. Percent WDC was also significantly lower (P < 0.05) in fallows than in continuous maize, and for the 0–5 cm it was 11, 10, 8 and 17 for A. angustissima, S. sesban, NF and continuous maize, respectively at fallow termination. After 2 years of cropping WDC (%) was 12, 14, 15 and 17 under CT and 10, 12, 12 and 16 under NT for A. angustissima, S. sesban, NF and continuous maize, respectively. MWD also showed significantly higher (P < 0.05) aggregate stability in fallows than in continuous maize. Water infiltration rates were significantly greater under fallows than continuous maize but these declined significantly during the cropping phase in plots that had been fallowed. In October 2000, infiltration rates in the A. angustissima and NF plots were above 35 mm h⁻¹ as no runoff was observed. Steady-state infiltration rates were 24 mm h⁻¹ in S. sesban and 5 mm h⁻¹ for continuous maize after 30 min of rainfall simulations. After 2 years of cropping infiltration rates remained above 35 mm h⁻¹ in A. angustissima plots, but declined to 18 and 8 mm h⁻¹ for NF, CT and NT respectively and 12 mm h⁻¹ for S. sesban, CT and NT. It is concluded that legume tree fallows improved SOC, aggregate stability and infiltration rates, but these benefits accrued during fallowing decreased significantly after 2 years of cropping following the termination of fallows. The decrease in SOC and aggregate stability was higher under CT than NT. Coppicing fallows of A. angustissima were the best long-term fallow species when integrated with NT as improved soil physical properties were maintained beyond 2 years of post-fallow cropping.     

Soil structure and organic carbon relationships following 10 years of wheat straw management in no-till

Crop residue retention is important for sequestering soil organic carbon (SOC), controlling soil erosion, and improving soil quality. Magnitude of residue management impacts on soil structural properties and SOC sequestration is, however, site specific. This study assessed long-term (10 year) impacts of three levels (0, 8, and 16 Mg ha⁻¹ on a dry matter basis) of wheat (Triticum aestivum L.) straw applied annually on SOC concentration and physical properties of the bulk soil and individual 5- to 8-mm aggregates for the 0- to 50-cm soil depth under no-till (NT) on a Crosby silt loam (fine, mixed, active, mesic Aeric Epiaqualfs) in central Ohio. This study also quantified relationships between soil properties and straw-induced changes in SOC concentration. Changes in soil properties due to straw mulching were mostly confined to the upper 5 cm of the soil. Mulching increased SOC concentration, but it did not significantly change cone index (CI) and shear strength (SHEAR). Within the upper 0–5-cm soil depth, mulching decreased bulk density (ρ_b) by 40–50%, aggregate density (ρ_agg) by 30–40%, and particle density (ρ_s) by 10–15%, and increased tensile strength (TS) of aggregates by up to 14 times as compared to unmulched soil. At the same depth, soil with mulch retained >30% more water than soil without mulch from 0 to −1500 kPa potentials. The SOC amount was 16.0 Mg ha⁻¹ under no straw, 25.3 Mg ha⁻¹ under 8 Mg ha⁻¹ straw, and 33.5 Mg ha⁻¹ under 16 Mg ha⁻¹ straw in the 0- to 10-cm depth. Below 10 cm, differences in SOC pool between mulched and unmulched soil were not significant. Overall, SOC from 0- to 50-cm depth was 82.5 Mg ha⁻¹ for unmulched soil, 94.1 Mg ha⁻¹ for 8 Mg ha⁻¹ mulch, and 104.9 Mg ha⁻¹ for 16 Mg ha⁻¹. About 33% of C added with straw over the 10-year period was sequestered in soil. This means that 2/3 of the wheat straw applied was not converted to SOC and most probably was lost as emissions of CO₂ and CH₄. The annual rate of total C accrual was 1.2 Mg ha⁻¹ in soil mulched with 8 Mg ha⁻¹ and 2.2 Mg ha⁻¹ in soil mulched with 16 Mg ha⁻¹ of straw in the 0- to 50-cm depth. The percentage of macroaggregates (>5-mm) was six times higher under 8 Mg ha⁻¹ of straw and 12 times higher under 16 Mg ha⁻¹ compared to unmulched treatments. Macroaggregates contained greater SOC than microaggregates in mulched soil. The SOC concentration explained the variability in aggregate properties by as much as 96%. Overall, long-term straw mulching increased SOC concentration and improved near-surface aggregate properties.     

Influence of temperature and drought on seasonal and interannual variations of soil, bole and ecosystem respiration in a boreal aspen stand

Continuous half-hourly measurements of soil (R_s) and bole respiration (R_b), as well as whole-ecosystem CO₂ exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (R_e) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ during winter to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-summer. Seasonal variations of R_s were highly correlated with variations of soil temperature (T_s) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean R_s from 2001 to 2003. Aspen daily mean R_b varied from negligible during winter to a maximum of 2.5 μmol m⁻² bark s⁻¹ (2.2 μmol m⁻² ground s⁻¹) during the growing season. Maximum R_b occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (T_b) in 2001 and 2003. Nonetheless, R_b was highly correlated with T_b and this variable explained 77, 87 and 62% of the variance in R_b in the respective years. Partitioning of R_b between its maintenance (R_bm) and growth (R_bg) components using the mature tissue method showed that daily mean R_bg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of R_bm and R_bg, respectively, during each year. Annual totals of R_s, R_b and estimated foliage respiration (R_f) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m⁻² year⁻¹, respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of R_e from chamber measurements. Scaled R_e estimates were 25% higher (1190 g C m⁻² year⁻¹) than the annual totals of R_e obtained from EC (949 g C m⁻² year⁻¹). The independent effects of temperature and drought on annual totals of R_e and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of R_s to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of R_s and R_e.     

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO₂ efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (R_s) on soil temperature (T_s) and water content (θ). Daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ in February to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-July. Daily mean T_s at the 2-cm depth was the primary variable accounting for the temporal variation of R_s and no differences between Arrhenius and Q₁₀ response functions were found to describe the seasonal relationship. R_s at 10 °C (R_s10) and the temperature sensitivity of R_s (Q_10Rs) calculated at the seasonal time scale were 3.8 μmol m⁻² s⁻¹ and 3.8, respectively. Temperature normalization of daily mean R_s (R_sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R_s during the growing season. Daily R_sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θ_fc, 0.30 m³ m⁻³): (1) R_sN was strongly reduced when θ decreased below θ_fc, which reflected a reduction in microbial decomposition, and (2) R_sN slightly decreased when θ increased above θ_fc, which reflected a restriction of CO₂ or O₂ transport in the soil profile.Diurnal variations of half-hourly R_s were usually out of phase with T_s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R_s10 and Q_10Rs parameters calculated from half-hourly nighttime measurements of R_s and T_s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m⁻² s⁻¹ and 5.5 to 1.3, respectively. On average, daily nighttime R_s10 (4.5 μmol m⁻² s⁻¹) and Q_10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R_s10 and Q_10Rs values at low θ. Overall, the use of seasonal R_s10 and Q_10Rs parameters led to an overestimation of daily ranges of half-hourly R_s (ΔR_s) during drought conditions, which supported findings that the short-term temperature sensitivity of R_s was lower during periods of low θ. The use of daily nighttime R_s10 and Q_10Rs parameters greatly helped at simulating ΔR_s during these periods but did not improve the estimation of half-hourly R_s throughout the year as it could not account for the diurnal hysteresis effect.     

Temporal and spatial variability of soil respiration in a boreal mixedwood forest

Temporal and spatial variability of soil respiration (R_s) was measured and analyzed in a 74-year-old, mixedwood, boreal forest in Ontario, Canada, over a period of 2 years (August 2003–July 2005). The ranges of R_s measured during the two study years were 0.5–6.9 μmol CO₂ m⁻² s⁻¹ for 2003–2004 (Year 1) and 0.4–6.8 μmol CO₂ m⁻² s⁻¹ for 2004–2005 (Year 2). Mean annual R_s for the stand was the same for both years, 2.7 μmol CO₂ m⁻² s⁻¹. Temporal variability of R_s was controlled mainly by soil temperature (T_s), but soil moisture had a confounding effect on T_s. Annual estimates of total soil CO₂ emissions at the site, calculated using a simple empirical R_s–T_s relationship, showed that R_s can account for about 88 ± 27% of total annual ecosystem respiration at the site. The majority of soil CO₂ emissions came from the upper 12 to 20 cm organic LFH (litter–fibric–humic) soil layer. The degree of spatial variability in R_s, along the measured transect, was seasonal and followed the seasonal trend of mean R_s: increasing through the growing season and converging to a minimum in winter (coefficient of variation (CV) ranged from 4 to 74% in Year 1 and 4 to 62% in Year 2). Spatial variability in R_s was found to be negatively related to spatial variability in the C:N ratio of the LHF layer at the site. Spatial variability in R_s was also found to depend on forest tree species composition within the stand. R_s was about 15% higher in a broadleaf deciduous tree patch compared to evergreen coniferous area. However, the difference was not always significant (at 95% CI). In general, R_s in the mixedwood patch, having both deciduous and coniferous species, was dominated by broadleaf trees, reflecting changing physiological controls on R_s with seasons. Our results highlight the importance of discerning soil CO₂ emissions at a variety of spatial and temporal scales. They also suggest including the LFH soil layer and allowing for seasonal variability in CO₂ production within that layer, when modeling soil respiration in forest ecosystems.     

Interactions between soil properties and moisture content in crust formation, runoff and interrill erosion from tilled loess soils

Soil-surface seals and crusts resulting from aggregate breakdown reduce the soil infiltration rate and may induce erosion by increasing runoff. The cultivated loess areas of northwestern Europe are particularly prone to these processes.Surface samples of ten tilled silty loamy loess soils, ranging in clay content from 120 to 350 g kg⁻¹ and in organic carbon from 10 to 20 g kg⁻¹, were packed into 0.5 m² plots with 5% slopes and subjected to simulated rainfall applied at 30 mm h⁻¹. The 120 minutes rainfall events were applied to initially field-moist soil, air-dried soil and rewetted soil to investigate the effect of soil moisture content prior to rainfall. Runoff and eroded sediments were collected at 5 minutes intervals. Aggregate stability of the soils was assessed by measuring particle-size distribution after different treatments.All soils formed seals. Runoff rates were between 70 and 90% by the end of the rainfall event for field-moist plots. There were large differences between soil runoff rates for the air-dried and rewetted plots. Interrill erosion was associated with runoff, and sediment concentration in runoff readily reached a steady-state value. Measurements of aggregate stability for various treatments were in good agreement with sealing, runoff and erosion responses to rainfall. Runoff and erosion were lower for air-dried plots than for field-moist plots, and were either intermediate or lowest for rewetted plots, depending on soil characteristics. Soils with a high clay content had the lowest erosion rate when they were rewetted, whereas the soil with a high organic-carbon content had the lowest erosion rate in air-dry conditions. The results indicate the complexity of the effect of initial moisture content, and the interactions between soil properties and climate.     

Tillage and drainage impact on soil quality: I. Aggregate stability, carbon and nitrogen pools

Effects of two tillage treatments, tillage (T) with chisel plough and no-till (NT), were studied under un-drained and drained soil conditions. Soil physical properties measured were bulk density (ρ_b), total porosity (ƒ_t), water stable aggregates (WSA), geometric mean diameter (GMD), mean weight diameter (MWD), organic carbon (OC) and total N concentrations in different aggregate size fractions, and total OC and N pools. The experiment was established in 1994 on a poorly drained Crosby silt loam soil (fine mixed, mesic, Aeric Ochraqualf) near Columbus, Ohio. In 2007, soil samples were collected (0–10, 10–20, and 20–30 cm) from all treatments and separated into six aggregate size classes for assessing proportions of macro (5–8, 2–5, 1–2, 0.5–1, 0.25–0.5) and micro (<0.25 mm) aggregates by wet sieving. Tillage treatments significantly (P ≤ 0.05) influenced WSA, MWD, and GMD. Higher total WSA (78.53 vs. 58.27%), GMD (0.99 vs. 0.68 mm), and MWD (2.23 vs. 0.99 mm) were observed for 0–10 cm depth for NT than T treatments. Relative proportion of macro-aggregates (>0.25-mm) was also more in NT than T treatment for un-drained plots. Conversely, micro-aggregates (<0.25-mm) were more in T plots for both drained and un-drained treatments. The WSA, MWD and GMD decreased with increase in soil depth. The OC concentration was significantly higher (P ≤ 0.05) in NT for un-drained (P ≤ 0.01) treatment for all soil depths. Within macro-aggregates, the maximum OC concentrations of 1.91 and 1.75 g kg⁻¹ in 1–2 mm size fraction were observed in NT for un-drained and drained treatments, respectively. Tillage treatments significantly (P < 0.01) affected bulk density (ρ_b), and total porosity (f_t) for all soil depths, whereas tillage × drainage interaction was significant (P < 0.01) for 10–20 and 20–30 cm depths. Soil ρ_b was negatively correlated (r = −0.47; n = 12) with OC concentration. Tillage treatments significantly affected (P ≤ 0.05) OC pools at 10–20 cm depth; whereas drainage, and tillage × drainage significantly (P ≤ 0.05) influenced OC pools for 0–10 cm soil layer. The OC pool in 0–10 cm layer was 31.8 Mg ha⁻¹ for NT compared with 25.9 Mg kg⁻¹ for T for un-drained treatment. In comparison, the OC pool was 23.1 Mg ha⁻¹ for NT compared with 25.2 Mg ha⁻¹ for T for the drained plots. In general, the OC pool was higher in NT system, coupled with un-drained treatment than in drained T plots. The data indicate the importance of NT in improving the OC pool.     

Relationship of percolation stability of soil aggregates to land use, selected properties, structural indices and simulated rainfall erosion

Simple tests of structural stability are needed for evaluating the ease with which soils slake and erode when in contact with water. In a laboratory study, we related the percolation stability (PS) of 22 Nigerian soils to land use, soil properties, structural stability indices and simulated rainfall erosion. All measurements were carried out with the 1–2 mm diameter air-dry aggregates. Land use influenced PS more than the type of soil. Forest soils, bush fallows, mulched, minimally tilled plots and pasture lands had rapid PS (>250 ml/10 min) values, whereas mulched conventionally tilled plots, bare fallows and continuously cultivated plots from where residues were removed by burning had relatively slow to moderate PS values (34–241 ml/10 min). The single most important soil property that correlated positively with PS is organic matter (OM) (r = 0.55^*) followed by total Fe + Al (r = 0.52^*). The significant inverse relationship (r = −0.49^*) between log (PS) and log (pH/OM) indicates a decrease in PS of these acidic, low-OM soils with increasing pH levels. The percent water-stable aggregate (WSA) >0.20 mm diameter, aggregated clay index (AC) and clay dispersion ratio (CDR) correlated weakly with PS. Conversely, the sealing index (SI) (i.e. the ratio of saturated hydraulic conductivity of an uncrusted to that of a crusted soil) had a strong, inverse relationship with PS (r = −0.97^***). These relationships indicate that PS measures the slakability (and not dispersibility) of soils. The relationship between PS and erosion (E) was an exponential decay form, E = 102 e^−0.0043PS (r² = 0.98) and showed that high interrill erosion rates would be expected on soils with PS < 250 ml/10 min. The PS which is simple to measure, is, therefore, a good indicator of structural stability for assessing the potential of these soils to erode.     

Tillage erosion within potato production in Atlantic Canada: II: Erosivity of primary and secondary tillage operations

To date, tillage erosion experiments in Canada have only been conducted on conventionally tilled corn-based production systems in Ontario and conventionally tilled cereal-based production in Manitoba. Estimates and assumptions have been made for all other production systems. Therefore, the objective of this study was to evaluate the erosivity of primary and secondary tillage operations within conventional and conservation potato production systems used in Atlantic Canada. Regression analysis determined that a direct relationship exists between slope gradient and both the mean displacement distance of the tilled layer (T_L) and the mass of translocated soil (T_M) for the chisel plough (CP), mouldboard plough (MP) and offset disc (OD), but not for the vibrashank (VS). Overall, the potential for tillage erosion of the MP, CP, and OD was similar (1.8–1.9 kg m⁻¹ %⁻¹ pass⁻¹) and larger than that of the VS (0.3 kg m⁻¹ %⁻¹ pass⁻¹). The regression coefficients for each implement were improved after including slope curvature, and we recommend that curvature be included in any future tillage erosion modelling. Our results show that both residue management to control wind and water erosion and soil movement to control tillage erosion must be considered when choosing implements and developing best management practices with regards to reducing the negative impacts of total soil erosion on potato production systems in Atlantic Canada.     

Friction velocity and aerodynamic roughness of conventional and undercutter tillage within the Columbia Plateau, USA

Friction velocity (u_*) and aerodynamic roughness (z_o) at the soil–plant–atmosphere interface affect wind erosion, but no attempts have been made to quantify these parameters as affected by tillage systems within the Columbia Plateau region of the Pacific Northwest United States. Wind velocity profiles above adjacent field plots (>2 ha), with plots subject to conventional or undercutter tillage during the summer fallow phase of a winter wheat–summer fallow rotation, were measured over 50 high wind events (wind velocities in excess of 6.4 m s⁻¹ at a height of 3 m) during 2005 and 2006 near Lind, Washington to determine u_* and z_o of tillage treatments. Wheat stubble plots were subject to either conventional (disks) or undercutter (wide V-shaped blades) tillage in spring and then periodically rodweeded prior to sowing winter wheat in late summer. Prior to sowing, u_* for conventional and undercutter tillage respectively averaged 0.36 and 0.46 m s⁻¹ in 2005 and 0.38 and 0.40 m s⁻¹ in 2006 while z_o for conventional and undercutter tillage respectively averaged 2 and 7 mm in 2005 and 2 and 4 mm in 2006. The aerodynamically rougher surface of undercutter tillage was predicted to suppress vertical dust flux; this was collaborated with observations in the field where undercutter tillage reduced dust flux as compared with conventional disk tillage. Undercutter tillage, therefore, appears to be an effective management practice to roughen the surface and thereby suppress dust emissions from agricultural land subject to summer fallow within the Columbia Plateau.     

Distillery effluents effect on soil organic carbon and aggregate stability of a Vertisol in India

Distillery effluent, a waste by-product of distillery industries, is usually applied to arable land near the distilleries as irrigation water or as a soil amendment. To evaluate the effect of distillery effluent, both spent wash (SW) and post-methanated effluent (PME), on soil organic carbon and aggregate stability, a field experiment on a soybean (Glysine max L.)–wheat (Triticum aestivum L.) system was conducted for five years on a Vertisol of central India. The treatments were control (no fertilizer or manure or SW or PME, T₁), 100% NPK + farmyard manure (FYM) @ 4 Mg ha⁻¹ to soybean (T₂), four graded levels of SW, viz., 2.5 cm SW to soybean and none to wheat (T₃), 2.5 cm SW to soybean and 1.25 cm to wheat (T₄), 5 cm SW to soybean and none to wheat (T₅), 5 cm SW to soybean and 2.5 cm to wheat (T₆), and four graded levels of PME, viz., 2.5 cm PME to soybean and none to wheat (T₇), 2.5 cm PME to soybean and 1.25 cm to wheat (T₈), 5 cm PME to soybean and none to wheat (T₉), 5 cm PME to soybean and 2.5 cm to wheat (T₁₀). The organic carbon of the surface (0–15 cm) soil that received either PME or SW (treatments T₃–T₁₀), was significantly (P < 0.05) higher than in treatments T₁ and T₂. The mean weight diameter (MWD) of water stable aggregates in this soil layer was also significantly higher in treatments T₃–T₁₀, compared with T₁ and T₂. The MWD showed a positive linear relationship with the organic carbon content of the soil (R² = 0.54**). The proportion of macro-aggregates was higher in SW treated plots than PME, no distillery effluents and NPK + FYM treatments. However, the micro-aggregates showed the reverse trend. The macro-aggregate-associated carbon was higher in SW treated plots. It was highest in T₆ and lowest in T₁. The plots receiving the PME and SW showed increased soil organic carbon, MWD, percentage macro- and micro-aggregate-associated carbon than T₁ and T₂. Application of distillery effluents increased the aggregate stability of the Vertisol through enhanced soil organic carbon as well as the aggregate-associated carbon. So application of SW or PME could be a viable option for soil aggregate stability and enhanced productivity.