Depth distribution of soil organic C and N after long-term soybean cropping in Texas

Crop management practices have potential to enhance subsoil C and N sequestration in the southern U.S., but effects may vary with tillage regime and cropping sequence. The objective of this study was to determine the impacts of tillage and soybean cropping sequence on the depth distribution of soil organic C (SOC), dissolved organic C (DOC), and total N after 20 years of treatment imposition for a silty clay loam soil in central Texas. A continuous soybean monoculture, a wheat–soybean doublecrop, and a sorghum–wheat–soybean rotation were established under both conventional (CT) and no tillage (NT). Soil was sampled after soybean harvest and sectioned into 0–5, 5–15, 15–30, 30–55, 55–80, and 80–105 cm depth intervals. Both tillage and cropping intensity influenced C and N dynamics in surface and subsurface soils. No tillage increased SOC, DOC, and total N compared to CT to a 30 cm depth for continuous soybean, but to 55 cm depths for the more intensive sorghum–wheat–soybean rotation and wheat–soybean doublecrop. Averaged from 0 to 105 cm, NT increased SOC, DOC, and total N by 32, 22, and 34%, respectively, compared to CT. Intensive cropping increased SOC and total N at depths to 55 cm compared to continuous soybean, regardless of tillage regime. Continuous soybean had significantly lower SOC (5.3 g kg⁻¹) than sorghum–wheat–soybean (6.4 g kg⁻¹) and wheat–soybean (6.1 g kg⁻¹), and 19% lower total N than other cropping sequences. Dissolved organic C was also significantly higher for sorghum–wheat–soybean (139 mg C kg⁻¹) than wheat–soybean (92 mg C kg⁻¹) and continuous soybean (100 mg C kg⁻¹). The depth distribution of SOC, DOC, and total N indicated treatment effects below the maximum tillage depth (25 cm), suggesting that roots, or translocation of dissolved organic matter from surface soils, contributed to higher soil organic matter levels under NT than CT in subsurface soils. High-intensity cropping sequences, coupled with NT, resulted in the highest soil organic matter levels, demonstrating potential for C and N sequestration for subsurface soils in the southern U.S.     

Long-term manuring and fertilization effects on soil organic carbon pools in a Typic Haplustept of semi-arid sub-tropical India

Soil is a potential C sink and could offset rising atmospheric CO₂. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Management practices, such as no-tillage and high intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. An investigation was carried out to study the influence of long-term applications of fertilizers and manures on different organic C fractions in a Typic Haplustept under intensive sequence of cropping with maize–wheat–cowpea in a semi-arid sub-tropic of India. In 0–15 cm, the bulk density was lowest (1.52 Mg m⁻³) in plots treated with 100% NPK + FYM, while the control treatment showed the highest value (1.67 Mg m⁻³). Balanced application of NPK (100% NPK) showed significantly lower bulk density (1.56 Mg m⁻³) over either 100% N (1.67 Mg m⁻³) or 100% NP (1.61 Mg m⁻³) in surface soils. The application of super-optimal dose of NPK (150% NPK) showed higher total organic C (TOC) (12.9 g C kg⁻¹) over either 50% NPK (9.3 g C kg⁻¹) or 100% NPK (10.0 g C kg⁻¹) in 0–15 cm soil layer. There was an improvement in TOC in 100% NPK or 100% NP (9.3 g C kg⁻¹) over 100% N (8.7 g C kg⁻¹) in the same depth. The application of FYM with 100% NPK showed 15.2, 9.9 and 5.2 g C kg⁻¹ in 0–15, 15–30 and 30–45 cm, respectively. Application of graded doses of NPK from 50 to 150% of recommendation NPK significantly enhanced other organic C fractions like, microbial biomass C (MBC), particulate organic C (POC) and KMnO₄ oxidizable C (KMnO₄–C) in all the three soil depths. The TOC in 0–45 cm soil depth in 150% NPK (63.5 Mg C ha⁻¹) was increased by 39% over that in 50% NPK treatment (51.5 Mg C ha⁻¹) and 29% over that in 100% NPK treatment (54.1 Mg C ha⁻¹). Integrated use of farmyard manure with 100% NPK (100% NPK + FYM) emerged as the most efficient management system in accumulating largest amount of organic C (72.1 Mg C ha⁻¹) in soil. Nevertheless, this treatment also sequestered highest amount of organic C (731 kg C ha⁻¹ year⁻¹). Particulate organic carbon, a physically protected carbon pool in soil, could well be protected in sub-surface soil layers than in surface soil layer as a means of carbon aggradations. Microbial metabolic quotient (qCO₂) was significantly lower in 100% NPK + FYM over other treatments to indicate this to be the most efficient manuring practice to preserve organic carbon in soil where it facilitates aggradations of more recalcitrant organic C in soil. As compared to POC, total TOC proved to be a better predictor of MBC as it strongly correlated with total carbon mineralized from soil.     

Rotation and tillage effects on soil organic carbon sequestration in a typic Hapludalf in Southern Ontario

Increased use of conservation tillage is being considered as a way to sequester atmospheric C in the soil. However, little information exists on the effect of rotation and its interaction with tillage on soil organic carbon (SOC). A research trial with combinations of rotations and tillage treatments was sampled 20 years after its establishment to assess the effects on SOC sequestration in a typic Hapludalf in southern Ontario, Canada. The cropping treatments included continuous corn (zea mays L.), six rotations comprised of 2 years of corn following 2 years of another crop or crop sequence, and continuous alfalfa (Medicago sativa L.). Each rotation was split into either fall moldboard plow (MP) or fall chisel plow (CP) treatments. Continuous alfalfa was plowed and replanted every 4 years. Soil samples were taken incrementally to a depth of 40 cm and SOC and bulk density determined. The average SOC concentration (0–40 cm) was greatest in continuous alfalfa (18.0 g C kg⁻¹). The treatments of soybean (Glycine max L.Merr.)+winterwheat (Triticum aestivum L.) or barley+barley (Trifolium pratense L.) (interseeded with red clover) followed by 2 years of corn had higher SOC concentrations (17.2–17.3 g C kg⁻¹) than continuous corn and the treatments of 2 years of corn following 2 years of alfalfa or soybean (16.4–16.5 g C kg⁻¹). The rotation of 2 years of barley followed by 2 years of corn had the lowest SOC concentrations (15.2 g C kg⁻¹). On an equivalent mass basis, the rotations of soybean+winterwheat or barley+barley (underseeded with red clover) followed by 2 years of corn, had 2–9 Mg ha⁻¹ more C than the other corn-based rotations. Including red clover in the winter wheat seemed to accelerate the rate of C mineralization compared to winter wheat without red clover; whereas interseeding red clover with barley increased SOC contents compared to excluding red clover in the barley rotation. More SOC was found in the top 10 cm and less in the 10–20 cm depth of the CP than in the MP soils. However, the CP did not increase the SOC content (0–20 cm) above that of MP indicating that this form of reduced tillage did not increase C sequestration in any of the rotations on this soil.     

Tillage, crop residue and N fertilizer effects on crop yield, nutrient uptake, soil quality and nitrous oxide gas emissions in a second 4-yr rotation cycle

An 8-yr (1998–2005) field experiment was conducted on a Gray Luvisol (Boralf) soil near Star City, Saskatchewan, Canada, to determine the effects of tillage (no-tillage – NT and conventional tillage – CT), straw management (straw retained – R and straw not retained – NR) and N fertilizer (0, 40, 80 and 120 kg N ha⁻¹, except no N to pea (Pisum sativum L.) phase of the rotation) on seed and straw yield, mass of N and C in crop, organic C and N, inorganic N and aggregation in soil, and nitrous oxide (N₂O) emissions for a second 4-yr rotation cycle (2002–2005). The plots were seeded to barley (Hordeum vulgare L.) in 2002, pea in 2003, wheat (Triticum aestivum L.) in 2004 and canola (Brassica napus L.) in 2005. Seed, straw and chaff yield, root mass, and mass of N and C in crop increased with increasing N rate for barley in 2002, wheat in 2004 and canola in 2005. No-till produced greater seed (by 51%), straw (23%) and chaff (13%) yield of barley than CT in 2002, but seed yield for wheat in 2004, and seed and straw yield for canola in 2005 were greater under CT than NT. Straw retention increased seed (by 62%), straw (by 43%) and chaff (by 12%) yield, and root mass (by 11%) compared to straw removal for barley in 2002, wheat in 2004, and seed and straw yield for pea in 2003. No-till resulted in greater mass of N in seed, and mass of C in seed, straw, chaff and root than CT for barley in 2002, but mass of N and C were greater under CT than NT for wheat in 2004 and for canola in 2005 in many cases. Straw retention had greater mass of N and C in seed, straw, chaff and root in most cases compared to straw removal for barley in 2002, pea in 2003 and wheat in 2004. Soil moisture content in spring was higher under NT than CT and with R than NR in the 0–15 cm depth, with the highest moisture content in the NT + R treatment in many cases. After eight crop seasons, tillage and straw management had no effect on total organic C (TOC) and N (TON) in the 0–15 cm soil, but light fraction organic C (LFOC) and N (LFON), respectively, were greater by 1.275 Mg C ha⁻¹ and 0.031 Mg N ha⁻¹ with R than NR, and also greater by 0.563 Mg C ha⁻¹ and 0.044 Mg N ha⁻¹ under NT than CT. There was no effect of tillage, straw and N fertilization on the NH₄-N in soil in most cases, but R treatment had higher NO₃-N concentration in the 0–15 cm soil than NR. The NO₃-N concentration in the 0–15, 15–30 and 30–60 cm soil layers increased (though small) with increasing N rate. The R treatment had 6.7% lower proportion of fine (<0.83 mm diameter) and 8.6% greater proportion of large (>38.0 mm) dry aggregates, and 4.5 mm larger mean weight diameter (MWD) compared to NR treatment. This suggests a lower potential for soil erosion when crop residues are retained. There was no beneficial effect of elimination of tillage on soil aggregation. The amount of N lost as N₂O was higher from N-fertilized (580 g N ha⁻¹) than from zero-N (155 g N ha⁻¹) plots, and also higher in CT (398 g N ha⁻¹) than NT (340 g N ha⁻¹) in some cases. In conclusion, retaining crop residues along with no-tillage improved some soil properties and may also be better for the environment and the sustainability of high crop production. Nitrogen fertilization improved crop production and some soil quality attributes, but also increased the potential for NO₃-N leaching and N₂O-N emissions, especially when applied in excess of crop requirements.     

Response of soil physical properties to tillage and residue management on two soils in a cool temperate environment

In view of their potential benefits, reduced or no tillage (NT) systems are being advocated worldwide. Concerns about impairment of some soil conditions, however, cast doubt on their unqualified acceptance. We evaluated the effects of 6 years of tillage and residue management on bulk density, penetration resistance, aggregation and infiltration rate of a Black Chernozem at Innisfail (loam, 65 g kg⁻¹ organic matter, Udic Boroll) and a Gray Luvisol at Rimbey (loam, 31 g kg⁻¹ organic matter, Boralf) cropped to monoculture spring barley (Hordeum vulgare L.) in a cool temperate climate in Alberta, Canada. Tillage systems were no tillage and tillage with rototilling (T), and two residue levels were straw removed (−S) and straw retained (+S). Bulk density (BD) of the 0–7.5 and 7.5–15 cm depths was significantly greater under NT (1.13–1.58 Mg m⁻³) than under T (0.99–1.41 Mg m⁻³) in both soils, irrespective of residue management. In both soils, penetration resistance (PR) was greater under NT than under T to 15 cm depth. Residue retention significantly reduced PR of the 0–10 cm soil in NT, but not in T. In the 0–5 cm depth of the Black Chernozem, the >2 mm fraction of dry aggregates was highest under NT + S (72%), and lowest under T − S (50%). The wind-erodible fraction (dry aggregates <1 mm size) was smallest (18%) under NT + S and largest (39%) under T − S. Soil aggregation benefited more from NT than from residue retention. Proportion of wind-erodible aggregates was generally greater in the Gray Luvisol than in the Black Chernozem. In the Black Chernozem, steady-state infiltration rate (IR) was significantly lower (33%) under NT than under T. Residue retention improved IR in both NT and T. In the Gray Luvisol, IR was not significantly affected by tillage and residue management. Despite firmer soil, NT and residue retention are recommended to improve aggregation in the cool temperate region of Western Canada.     

Effects of mechanical energy inputs on soil respiration at the aggregate and field scales

Cultivation machinery applies large amounts of mechanical energy to the soil and often brings about a decrease in soil organic carbon (SOC). New experiments on the effects of mechanical energy inputs on soil respiration are reported and the results discussed. In the laboratory, a specific energy, K, of 150 J kg⁻¹, similar to that experienced during typical cultivation operations, was applied to soil aggregates using a falling weight. Respiration (carbon dioxide, CO₂ emission) of the samples was then measured by an electrical conductimetric method. Basal respiration (when K=0) measured on Chromic Luvisol aggregates, was found to increase with increasing SOC, from 1.88 μg CO₂ g⁻¹ h⁻¹ for a permanent fallow soil (SOC=11 g kg⁻¹) to 8.25 μg CO₂ g⁻¹ h⁻¹ for a permanent grassland soil (SOC=32 g kg⁻¹). Basal respiration of a Calcic Cambisol, more than doubled (2.0–5.2 μg CO₂ g⁻¹ h⁻¹) with increasing gravimetric soil water contents. Mechanical energy inputs caused an initial burst of increased respiration, which lasted up to 4 h. Over the following 4–24 h period, arable soils with lower SOC contents, (11–21 g kg⁻¹), respiration rates dropped back to a level, approximately 1.14 times higher than the basal value. However, grassland soils with higher SOC contents (28–32 g kg⁻¹), increases in this longer-term respiration rate following 150 J kg⁻¹ of energy, were negligible. A field experiment, in which CO₂ was measured by infra-red absorption, also showed that tillage stimulated increased levels of soil respiration for periods ranging from 12 h to more than one week. The highest respiration rates, 80 mg CO₂ m⁻² h⁻¹ were associated with high energy, powered tillage on clay soils. On the same soil, low energy draught tillage resulted in a respiration rate of approximately half this value. The results of these experiments are discussed in relation to equilibrium levels of soil organic matter. The application of known quantities of mechanical energy to soil aggregates under laboratory conditions, in order to simulate the effect of different cultivation practices, when combined with the subsequent measurement of soil respiration, can provide useful indication of the likely consequences of soil management on SOC.     

Effects of tillage and nitrogen management on soil chemical and physical properties after 23 years of continuous sorghum

Long-term tillage and nitrogen (N) management practices can have a profound impact on soil properties and nutrient availability. A great deal of research evaluating tillage and N applications on soil chemical properties has been conducted with continuous corn (Zea Mays L.) throughout the Midwest, but not on continuous grain sorghum (Sorghum bicolor (L.) Moench). The objective of this experiment was to examine the long-term effects of tillage and nitrogen applications on soil physical and chemical properties at different depths after 23 years of continuous sorghum under no-till (NT) and conventional till (CT) (fall chisel-field cultivation prior to planting) systems. Ammonium nitrate (AN), urea, and a slow release form of urea were surface broadcast at rates of 34, 67, and 135 kg N ha⁻¹. Soil samples were taken to a depth of 15 cm and separated into 2.5 cm increments. As a result of lime applied to the soil surface, soil pH in the NT and CT plots decreased with depth, ranging from 6.9 to 5.7 in the NT plots and from 6.5 to 5.9 in the CT plots. Bray-1 extractable P and NH₄OAc extractable K was 20 and 49 mg kg⁻¹ higher, respectively, in the surface 2.5 cm of NT compared to CT. Extractable Ca was not greatly influenced by tillage but extractable Mg was higher for CT compared to NT below 2.5 cm. Organic carbon (OC) under NT was significantly higher in the surface 7.5 cm of soil compared to CT. Averaged across N rates, NT had 2.7 Mg ha⁻¹ more C than CT in the surface 7.5 cm of soil. Bulk density (Δ_b) of the CT was lower at 1.07 g cm⁻³ while Δ_b of NT plots was 1.13 g cm⁻³. This study demonstrated the effect tillage has on the distribution and concentration of certain chemical soil properties.     

Cultivation effects on temporal changes of organic carbon and aggregate stability in desert soils of Hexi Corridor region in China

Sustainable agricultural use of cultivated desert soils has become a concern in Hexi Corridor in Gansu Province of China, because loss of topsoil in dust storms has been recently intensified. We chose four desert sites to investigate the effects of cultivation (cropping) on (i) soil organic C and its size fractions and (ii) soil aggregate stability (as a measure of soil erodibility). These parameters are of vital importance for evaluating the sustainability of agricultural practices.

Total organic C as well as organic C fractions in soil (coarse organic C, 0.1–2 mm; young organic C, 0.05–0.1 mm; stable organic C, <0.05 mm) generally increased with the duration of the cultivation period from 0 (virgin soil, non-cultivated) to more than 30 years (p < 0.05). Compared to total organic C in virgin soils (2.3–3.5 g kg⁻¹ soil), significantly greater values were found after 10 to >20 years of cultivation (6.2–7.1 g kg⁻¹ soil). The increase in organic C in desert soils following prolonged cultivation was mainly the consequence of an increase in the coarse organic C. The increase in total organic C in soil was also dependent on clay content [total organic C = 0.96 + 0.249 clay content (%) + 0.05 cultivation year, R² = 0.48, n = 27, p < 0.001]. This indicates that clay protected soil organic C from mineralization, and also contributed to the increase in soil organic C as time of cultivation increased.

There was a significant positive correlation between aggregate stability and total organic C across all field sites. The water stability of aggregates was low (with water-stable aggregate percentage 4% of dry-sieved aggregates of size 1–5 mm). There was no consistent pattern of increase in the soil aggregate stability with time of cultivation at different locations, suggesting that desert soils might remain prone to wind erosion even after 50 years of cultivation. Alternative management options, such as retaining harvested crop residues on soil surface and excluding or minimizing tillage, may permit sustainable agricultural use of desert soils.     

Depth distribution and bioavailability of pollutants in long-term differently tilled soils

Pollutants can be introduced to soil through the application of organic and inorganic fertilizers and pesticides and through atmospheric depositions. The objective of this research was to evaluate the influence of long-term (9–17 years) tillage systems on the behavior of pollutants in soils. Bioavailability and enrichment of heavy metals, arsenic, and organics, i.e. polychlorinated biphenyls (PCB’s) and a chlorinated phenol (2,4-DCP) were measured in a Eutric Cambisol and a Luvisol under conventional tillage (CT), reduced tillage (RT), and no-tillage (NT). Soil samples were collected from 0 to 3, 3 to 10, and 10 to 25 cm depths.

The upper layer of NT soils was enriched in pollutants, but concentrations decreased with increasing soil depth. Atmospheric deposition of pollutants and input via organic fertilizers was noticeable in soils under long-term NT. Total amount of zinc (59 mg kg⁻¹) was significantly enriched in the 0–3 cm depth of the Luvisol under NT and this was attributed to higher sorption capacity for heavy metal input via liquid manure. In the Eutric Cambisol, NT resulted in significant increase of cadmium extracted by aqua regia in the arable layer of 0–25 cm. As a result of higher soil organic C, long-term accumulation of PCB’s in NT soils was more pronounced than in plowed soils. In plowed soils the mixing effect resulted in homogeneous distribution of pollutants within a soil depth of 0–25 cm.

The enrichment of organic C in RT and NT soils emphasizes the role of soils as a sink for pollutants, buffering the contaminants against leaching and transfer into crops.     

Cultivation effects on biochemical properties, C storage and N natural abundance in the 0–5 cm layer of an acidic soil from temperate humid zone

Changes in some soil chemical, including ¹⁵N values, and biochemical properties (microbial C, FDA hydrolysis, glucosidase and urease activities) due to two tillage systems, conventional tillage (CT) and no-tillage (NT), were evaluated in an acid soil from temperate humid zone (NW of Spain) and compared with values obtained for a reference forest soil. The results showed that in the surface layer (0–5 cm depth) tillage tended to increase soil pH and to decrease organic matter levels and microbial biomass and activity values. The data also indicated that 8 years of NT, compared to CT, resulted in greater organic matter content and increased microbial biomass and activity, the changes being more pronounced for the microbial properties. Adoption of NT resulted in an increase of soil C storage of 1.24 Mg C ha⁻¹ year⁻¹ with regard to CT. The suitability of ¹⁵N as a potential tracer of land-use in this acid soil was also confirmed.     

Soil fertility properties on Agave angustifolia Haw. plantations

This study examined the variations in soil physical, chemical and biological properties from Agave angustifolia fields in three sites with different topographic conditions (valley, hill and mountain), in Oaxaca, Mexico, associated with the tillage systems, disk ploughing (DP), animal drawn ploughing (ADP) and minimum tillage (MT), respectively. Plant ages were 1.5–3.5 years (class 1), 3.6–5.5 years (class 2) and 5.6–7.5 years (class 3). Soil samples were taken at two soil depths (0–20 and 21–40 cm) from plots of 4000 m² within each site and plant age classes, during the spring of 2005. The main changes in soil properties were found in the mountain site. Soil bulk density (2.0 g cm⁻³), cone penetration resistance (CPR) (3.96 MPa), 0.7 and 1.0 mm water stable aggregates (WSA) (28.3 g kg⁻¹ and 102.2 g kg⁻¹, respectively) were higher in the mountain site than in the hill and valley fields. This result is consistent with the rocky substrate beneath the shallow soil. Soil organic carbon (SOC) (23.9 g kg⁻¹), available N (23.1 mg kg⁻¹) and soil microbial biomass carbon (SMBC) (969.6 μg g⁻¹) at the mountain site showed the highest values, suggesting that MT practiced in this topographic condition favours the organic matter accumulation and biological activity. Soil microbial biomass carbon and SOC seem to be the soil properties that were mainly affected by the sites and soil management associated with them. For the three sites, SOC, P_Olsen, available N, exchangeable Na⁺ and SMBC were higher at 0–20 cm depth than at 21–40 cm depth within each site. Exchangeable Ca²⁺ and K⁺, P_Olsen and CPR increased with plant age. In contrast, available N decreased. Soil chemical properties were more affected by the age of the plant than physical and biological properties. Results reported here represent a reference of the fertility properties of soils cultivated with A. angustifolia, which could be used in further studies focused on management and tillage systems.     

Erosional effects on soil organic carbon stock in an on-farm study on Alfisols in west central Ohio

Soil erosion and depositional processes in relation to land use and soil management need to be quantified to better understand the soil organic carbon (SOC) dynamics. This study was undertaken on a Miamian soil (Oxyaquic Hapludalfs) under on-farm conditions in western Ohio with the objectives of evaluating the effects of degree of erosion on SOC stock under a range of tillage systems. Six farms selected for this study were under: no-till (NT) for 15, 10, 6 and 1.5 years; chisel till every alternate year with annual manure application (MCT); and annual chisel till (ACT). A nearby forest (F) site on the same soil was chosen as control. Using the depth of A horizon as an indicator of the degree of erosion, four erosion phases identified were: uneroded (flat fields under F, NT15, and on the summit of sloping fields under NT10, NT6, NT1.5 and MCT); deposition (NT10, NT6, NT1.5 and ACT); slight (NT10, MCT and ACT); and moderate erosion (NT10 and ACT). Core and bulk soil samples were collected in triplicate from four depths (i.e., 0–10, 10–20, 20–30 and 30–50 cm) for each erosional phase in each field for the determination of bulk density, and SOC concentrations and stocks. SOC concentration in NT fields increased at a rate of 5% year⁻¹ for 0–10 cm and 2.5% year⁻¹ for 10–20 cm layer with increasing duration under NT. High SOC concentration for NT15 is indicative of SOC-sequestration potential upon conversion from plow till to NT. SOC concentration declined by 19.0–14.5 g kg⁻¹ in MCT and 11.3–9.7 g kg⁻¹ in NT10 between uneroded and slight erosion, and 12.0–11.2 g kg⁻¹ between slight and moderate erosion in ACT. Overall SOC stock was greatest in the forest for each of the four depths. Total SOC stock for the 50 cm soil layer varied in the order F (71.99 Mg ha⁻¹) > NT15 (56.10 Mg ha⁻¹) > NT10 (37.89 Mg ha⁻¹) = NT6 (36.58 Mg ha⁻¹) for uneroded phase (P < 0.05). The lack of uneroded phase in ACT indicated high erosion risks of tillage, as also indicated by the high SOC stock for deposition phase from 0 to 50 cm soil layer (ACT (56.56 Mg ha⁻¹) > NT1.5 (42.70 Mg ha⁻¹) > NT10 (30.97 Mg ha⁻¹)). Tillage increased soil erosion and decreased SOC stock for top 10 cm layer for all erosional phases except deposition.     

No-till effects on organic matter, pH, cation exchange capacity and nutrient distribution in a Luvisol in the semi-arid subtropics

No-till (NT) system for grain cropping is increasingly being practised in Australia. While benefits of NT, accompanied by stubble retention, are almost universal for soil erosion control, effects on soil organic matter and other soil properties are inconsistent, especially in a semi-arid, subtropical environment. We examined the effects of tillage, stubble and fertilizer management on the distribution of organic matter and nutrients in the topsoil (0–30 cm) of a Luvisol in a semi-arid, subtropical environment in southern Queensland, Australia. Measurements were made at the end of 9 years of NT, reduced till (RT) and conventional till (CT) practices, in combination with stubble retention and fertilizer N (as urea) application strategies for wheat (Triticum aestivum L.) cropping.

In the top 30 cm depth, the mean amount of organic C increased slightly after 9 years, although it was similar under all tillage practices, while the amount of total N declined under CT and RT practices, but not under NT. In the 0–10 cm depth, the amounts of organic C and total N were significantly greater under NT than under RT or CT. No-till had 1.94 Mg ha⁻¹ (18%) more organic C and 0.20 Mg ha⁻¹ (21%) more total N than CT. In the 0–30 cm depth, soil under NT practice had 290 kg N ha⁻¹ more than that under the CT practice, most of it in the top 10 cm depth. Microbial biomass N was similar for all treatments. Under NT, there was a concentration gradient in organic C, total N and microbial biomass N, with concentrations decreasing from 0–2.5 to 5–10 cm depths.

Soil pH was not affected by tillage or stubble treatments in the 0–10 cm depth, but decreased significantly from 7.5 to 7.2 with N fertilizer application. Exchangeable Mg and Na concentration, cation exchange capacity and exchangeable Na percentage in the 0–10 cm depth were greater under CT than under RT and NT, while exchangeable K and bicarbonate-extractable P concentrations were greater under NT than under CT.

Therefore, NT and RT practices resulted in significant changes in soil organic C and N and exchangeable cations in the topsoil of a Luvisol, when compared with CT. The greater organic matter accumulation close to the soil surface and solute movement in these soils under NT practice would be beneficial to soil chemical and physical status and crop production in the long-term, whereas the concentration of nutrients such as P and K in surface layers may reduce their availability to crops.     

The impact of tillage and residual nitrogen on wheat

Wheat (Triticum aestivum L.) yield and quality is influenced by management of the previous crop but is highly dependent on current year management. The objective of this study was to evaluate the response of winter wheat seeded in two tillage systems [conventional tillage (CT) and no-till (NT)] to four N rates applied to a previous cotton (Gossypium hirsutum L.) crop (0, 67, 134, and 202 kg ha⁻¹). The experiment with wheat was conducted on a Dothan sandy loam (fine, loamy siliceous, thermic Plinthic Kandiudults) at the University of Florida North Florida Research and Education Center near Quincy, FL from 1995 to 1997. For most plant characteristics, there was a tillage x N x year interaction. Greater plant emergence (79.4 vs. 65.3%) and grain N (23.5 vs. 21.5 g kg⁻¹), and lower grain moisture (139 vs. 142 g kg⁻¹) were obtained under NT than CT, respectively, in one out of two years. Nitrogen applied to a previous cotton crop increased wheat grain yields, plant height and seed number under NT in 1995–1996 and CT in 1996–1997, head density under NT in both years, harvest index under CT in 1996–1997, and grain N concentration in 1995–1996 and 1996–1997 due to residual plant and soil N. With every 1 kg N applied to a previous cotton crop, wheat grain yields increased by 5.38 kg ha⁻¹ under NT, whereas grain yield under CT was not influenced by N application to cotton in 1995–1996. In 1996–1997, grain yields increased by 4.96 and 4.23 kg ha⁻¹ for wheat grown in NT and CT, respectively. Generally, wheat seeded in NT following cotton did not decrease stand or yields compared to CT and wheat grain yields and grain N content increased with N fertilization of the previous crop. However, we would have to apply about 134 kg N ha⁻¹ to a previous cotton crop to maximize wheat production under NT and CT.     

Soil microbial biomass response to woody plant invasion of grassland

Woody plant proliferation in grasslands and savannas has been documented worldwide in recent history. To better understand the consequences of this vegetation change for the C-cycle, we measured soil microbial biomass carbon (C_mic) in remnant grasslands (time 0) and woody plant stands ranging in age from 10 to 130 years in a subtropical ecosystem undergoing succession from grassland to woodlands dominated by N-fixing trees. We also determined the ratio of SMB-C to soil organic carbon (C_mic/C_org) as an indicator of soil organic matter quality or availability, and the metabolic quotient (qCO₂) as a measure of microbial efficiency. Soil organic carbon (C_org) and soil total nitrogen (STN) increased up to 200% in the 0–15 cm depth increment following woody plant invasion of grassland, but changed little at 15–30 cm. C_mic at 0–15 cm increased linearly with time following woody plant encroachment and ranged from 400 mg C kg⁻¹ soil in remnant grasslands up to 600–1000 mg C kg⁻¹ soil in older (>60 years) woody plant stands. C_mic at 15–30 cm also increased linearly with time, ranging from 100 mg C kg⁻¹ soil in remnant grasslands to 400–700 mg C kg⁻¹ soil in older wooded areas. These changes in C_mic in wooded areas were correlated with concurrent changes in stores of C and N in soils, roots, and litter. The C_mic/C_org ratio at 0–15 cm decreased with increasing woody plant stand age from 6% in grasslands to <4% in older woodlands suggesting that woody litter may be less suitable as a microbial substrate compared with grassland litter. In addition, higher qCO₂ values in woodlands (0.8 mg CO₂-C g⁻¹ C_mic h⁻¹) relative to remnant grasslands (0.4 mg CO₂-C g⁻¹ C_mic h⁻¹) indicated that more respiration was required per unit of C_mic in wooded areas than in grasslands. Observed increases in C_org and STN following woody plant encroachment in this ecosystem may be a function of both greater inputs of poor quality C that is relatively resistant to decay, and the decreased ability of soil microbes to decompose this organic matter. We suggest that increases in the size and activity of C_mic following woody plant encroachment may result in: (a) alterations in competitive interactions and successional processes due to changes in nutrient dynamics, (b) enhanced formation and maintenance of soil physical structures that promote C_org sequestration, and/or (c) increased trace gas fluxes that have the potential to influence atmospheric chemistry and the climate system at regional to global scales.     

Soil organic carbon accumulation and carbon costs related to tillage, cropping systems and nitrogen fertilization in a subtropical Acrisol

Conservation management systems can improve soil organic matter stocks and contribute to atmospheric C mitigation. This study was carried out in a 18-year long-term experiment conducted on a subtropical Acrisol in Southern Brazil to assess the potential of tillage systems [conventional tillage (CT) and no-till (NT)], cropping systems [oat/maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/MC)] and N fertilization [0 kg N ha⁻¹ year⁻¹ (0 N) and 180 kg N ha⁻¹ year⁻¹ (180 N)] for mitigating atmospheric C. For that, the soil organic carbon (SOC) accumulation and the C equivalent (CE) costs of the investigated management systems were taken into account in comparison to the CT O/M 0 N used as reference system. No-till is known to produce a less oxidative environment than CT and resulted in SOC accumulation, mainly in the 0–5 cm soil layer, at rates related to the addition of crop residues, which were increased by legume cover crops and N fertilization. Considering the reference treatment, the SOC accumulation rates in the 0–20 cm layer varied from 0.09 to 0.34 Mg ha⁻¹ year⁻¹ in CT and from 0.19 to 0.65 Mg ha⁻¹ year⁻¹ in NT. However, the SOC accumulation rates peaked during the first years (5th to 9th) after the adoption of the management practices and decreased exponentially over time, indicating that conservation soil management was a short-term strategy for atmospheric C mitigation. On the other hand, when the CE costs of tillage operations were taken into account, the benefits of NT to C mitigation compared to CT were enhanced. When CE costs related to N-based fertilizers were taken into account, the increases in SOC accumulation due to N did not necessarily improve atmospheric C mitigation, although this does not diminish the agricultural and economic importance of inorganic N fertilization.     

Tillage-induced CO2 loss across an eroded landscape

Soil carbon (C) losses and soil translocation from tillage operations have been identified as causes of soil degradation and soil erosion. The objective of this work was to quantify the variability in tillage-induced carbon dioxide (CO₂) loss by moldboard (MP) and chisel (CP) plowing across an eroded landscape and relate the C loss to soil properties. The study site was a 4 ha wheat (Triticum aestivum L. cv. Marshall) field with rolling topography and five soil types in the Svea-Barnes complex in west central Minnesota (N. Latitude = 45°41′W, Longitude = 95°43′). Soil properties were measured at several depths at a 10 m spacing along north–south (N–S) and west–east (W–E) transects through severely eroded, moderately eroded and non-eroded sites. Conventional MP (25 cm deep) and CP (15 cm deep) equipment were used along the pre-marked transects. Gas exchange measurements were obtained with a large, portable chamber within 2 m of each sample site following tillage. The measured CO₂ fluxes were largest with the MP > CP > not tilled (before tillage). The variation in 24 h cumulative CO₂ flux from MP was nearly 3-fold on the N–S transect and 4-fold on the W–E transect. The surface soil organic C on the transects was lowest on the eroded knolls at 5.1 g C kg⁻¹ and increased to 19.6 g C kg⁻¹ in the depositional areas. The lowest CO₂ fluxes were measured from severely eroded sites which indicated that the variation in CO₂ loss was partially reflected by the degradation of soil properties caused by historic tillage-induced soil translocation with some wind and water erosion.

The spatial variation across the rolling landscape complicates the determination of non-point sources of soil C loss and suggests the need for improved conservation tillage methods to maintain soil and air quality in agricultural production systems.     

A short-term investigation of trace gas emissions following tillage and no-tillage of agroforestry residues in western Kenya

Improved-fallow agroforestry systems are increasingly being adopted in the humid tropics for soil fertility management. However, there is little information on trace gas emissions after residue application in these systems, or on the effect of tillage practice on emissions from tropical agricultural systems. Here, we report a short-term experiment in which the effects of tillage practice (no-tillage versus tillage to 15 cm depth) and residue quality on emissions of N₂O, CO₂ and CH₄ were determined in an improved-fallow agroforestry system in western Kenya. Emissions were increased following tillage of Tephrosia candida (2.1 g N₂O-N ha⁻¹ kg N applied⁻¹; 759 kg CO₂-C ha⁻¹ t C applied⁻¹; 30 g CH₄-C ha⁻¹ t C applied⁻¹) and Crotalaria paulina residues (2.8 g N₂O-N ha⁻¹ kg N applied⁻¹; 967 kg CO₂-C ha⁻¹ t C applied⁻¹; 146 g CH₄-C ha⁻¹ t C applied⁻¹) and were higher than from tillage of natural-fallow residues (1.0 g N₂O-N ha⁻¹ kg N applied⁻¹; 432 kg CO₂-C ha⁻¹ t C applied⁻¹; 14.7 g CH₄-C ha⁻¹ t C applied⁻¹) or from continuous maize cropping systems. Emissions from these fallow treatments were positively correlated with residue N content (r = 0.62–0.97; P < 0.05) and negatively correlated with residue lignin content (r = −0.56, N₂O; r = −0.92, CH₄; P < 0.05). No-tillage of surface applied Tephrosia residues lowered the total N₂O and CO₂ emitted over 99 days by 0.33 g N₂O-N ha⁻¹ kg N applied⁻¹ and 124 kg CO₂-C ha⁻¹ t C applied⁻¹, respectively; estimated to provide a reduction in global warming potential of 41 g CO₂ equivalents. However, emissions were increased from this treatment over the first 2 weeks. The responses to tillage practice and residue quality reported here need to be verified in longer term experiments before they can be used to suggest mitigation strategies appropriate for all three greenhouse gases.     

Transport of labile carbon in runoff as affected by land use and rainfall characteristics

The mobilization of organic carbon (C) by water erosion could impact the terrestrial C budget, but the magnitude and direction of that impact remain uncertain due to a lack of data regarding the fates and quality of eroded C. A study was conducted to monitor total organic C and mineralizable C (MinC) in eroded materials from watersheds under no till (NT), chisel till (CT), disk till low input (DT-LI), pasture and forest. The DT-LI treatment relies on manure application and legume cover crops to partly supply the N needed when corn is grown, and on cultivation to reduce the use of herbicides. Each watershed was instrumented with a flume and a Coshocton wheel sampler for runoff measurement. Carbon dioxide (CO₂) evolved during incubation (115 days) of runoff samples was fitted to a first-order decomposition model to derive MinC. Annual soil (6.2 Mg ha⁻¹) and organic C (113.8 kg C ha⁻¹) losses were twice as much in the DT-LI than in the other watersheds (<2.7 Mg soil ha⁻¹, <60 kg C ha⁻¹). More than management practices, rainfall class (based on intensity and energy) was a better controller of sediment C concentration and biodegradability. Sediment collected during the low-intensity (fall/winter) storms contained more organic C (37 g C kg⁻¹) and MinC (30–40% of sediment C) than materials displaced during the high-intensity summer storms (22.1 g C kg⁻¹ and 13%, respectively). These results suggest a more selective detachment and sorting of labile C fractions during low-intensity storms. However, despite the control of low-intensity storm on sediment C concentration and quality, increased soil loss with high-energy rainfall suggests that a few infrequent but high-energy storms could determine the overall impact of erosional events on terrestrial C cycling.     

Carbon sequestration in two Brazilian Cerrado soils under no-till

A considerable proportion of the 200 million hectares of the Brazilian Cerrado is suitable for annual crops but little is known about the effects of tillage on the C dynamics of Cerrado soils. We evaluated the role of two representative Cerrado Oxisols (350 and 650 g clay kg⁻¹) as sources or sinks of atmospheric C when managed under three tillage systems (conventional tillage (CT), reduced tillage (RT), and no-till (NT)) in 8- and 5-year long-term experiments. A literature review was also carried out and the mean C sequestration rates in no-till soils of tropical and subtropical regions of Brazil were calculated and compared with values for soils from temperate regions of the world. The original C stocks in 0–20 cm layer of soils under native Cerrado were higher in the clayey (54.0 Mg ha⁻¹) than in the sandy clay loam soil (35.4 Mg ha⁻¹), suggesting a higher physical stability of organic matter associated with variable clay minerals in the clayey Oxisol. The original C stocks of the native Cerrado soils appear not to have decreased after 23 years of conventional tillage in the sandy clay loam Oxisol, except when the soil had been subjected to erosion (15% loss of C), or after 25 years in the clayey Oxisol. Compared to conventionally tilled soil, the C stocks in no-till sandy clay loam Oxisol increased by 2.4 Mg ha⁻¹ (C sequestration rate = 0.30 Mg ha⁻¹ year⁻¹) and in the clayey Oxisol by 3.0 Mg ha⁻¹ (C sequestration rate = 0.60 Mg ha⁻¹ year⁻¹). The mean rate of C sequestration in the no-till Brazilian tropical soils was estimated to be 0.35 Mg ha⁻¹ year⁻¹, similar to the 0.34 Mg ha⁻¹ year⁻¹ reported for soils from temperate regions but lower than the 0.48 Mg ha⁻¹ year⁻¹ estimated for southern Brazilian subtropical soils. Considering the large area (about 70 million hectares) of the Cerrado which is currently used and potentially available for cropland, the adoption of no-till systems could turn the Cerrado soils into a significant sink for atmospheric C and contribute to the mitigation of global climate change.