Relationship between soil structure and runoff/soil loss after 24 years of conservation tillage

A better understanding of tillage and stubble management effects on surface soil structure is vital for the development of effective soil conservation practices for the long-term. Relationships between aspects of soil structure and runoff/soil loss were investigated in 24 year old field experiment on an Oxic Paleustalf, in NSW, Australia. Two tillage/stubble management systems were compared, namely direct drilled/stubble retained (DD/SR) versus conventional tillage/stubble burnt (CC/SB). Tillage and stubble burning significantly increased bulk density and decreased the macro-aggregate stability, mean weight diameter (MWD), geometric mean diameter (GMD) and total porosity, particularly macroporosity (>60 μm). For the 0–5 cm layer, DD/SR had significantly higher water stability of macro-aggregates >2 mm than CC/SB (165 g/kg versus 78 g/kg), and the volume of pore space of diameter >60 μm at 0–5 cm depth was significantly greater (more than 11%) for DD/SR than for CC/SB. Under simulated rainfall (100 mm/h) and the removal of surface stubble, both runoff and soil loss were significantly higher under CC/SB compared to DD/SR. The infiltration rate at the end of the experiment under DD/SR was 3.7 times that of CC/SB (85 mm/h versus 23 mm/h). There were significant correlations between the proportion of soil particles >0.25 mm measured after wetting by rain and both final infiltration rate (P < 0.001) and soil loss (P < 0.001). It was concluded that 24 years of direct drilling and stubble retained practices significantly reduced runoff and soil erosion hazards, due to a fundamental change in soil structure, viz. higher soil aggregate stability and higher macroporosity of the surface soil.     

Methane emission and entrapment in flooded rice soils as affected by soil properties

Laboratory incubation experiments were conducted to study the effects of soil chemical and physical properties on CH₄ emission and entrapment in 16 selected soils with a pH range of 4.7–8.1, organic matter content of 0.72–2.38%, and soil texture from silt to clay. There was no significant correlation with CH₄ emission for most of the important soil properties, including soil aerobic pH (measured before anaerobic incubation), total Kjeldahl N, cation exchange capacity, especially soil organic matter, and soil water-soluble C, which were considered to be critical controlling factors of CH₄ emission. A lower CH₄ emission was observed in some soils with a higher organic matter content. Differences in soil Fe and Mn contents and their chemical forms contributed to the this observation. A significant correlation between the CH₄ emission and the soil organic C content was observed only after stratifying soils into subgroups according to the level of CH₄ emission in soils not amended with organic matter. The results also showed that the soil redox potential (Eh), anaerobic pH, anerobic pH, and biologically reducible Fe and Mn affected CH₄ emission significantly. Urea fertilization promoted CH₄ emission in some soils and inhibited it in others. This result appeared to be related to the original soil pH. CH₄ entrapment was positively correlated with soil clay content, indicating the importance of soil physical characteristics in reducing CH₄ emissions to the atmosphere.     

Long-term impact of reduced tillage and residue management on soil carbon stabilization: Implications for conservation agriculture on contrasting soils

Residue retention and reduced tillage are both conservation agricultural management options that may enhance soil organic carbon (SOC) stabilization in tropical soils. Therefore, we evaluated the effects of long-term tillage and residue management on SOC dynamics in a Chromic Luvisol (red clay soil) and Areni-Gleyic Luvisol (sandy soil) in Zimbabwe. At the time of sampling the soils had been under conventional tillage (CT), mulch ripping (MR), clean ripping (CR) and tied ridging (TR) for 9 years. Soil was fully dispersed and separated into 212–2000 μm (coarse sand), 53–212 μm (fine sand), 20–53 μm (coarse silt), 5–20 μm (fine silt) and 0–5 μm (clay) size fractions. The whole soil and size fractions were analyzed for C content. Conventional tillage treatments had the least amount of SOC, with 14.9 mg C g⁻¹ soil and 4.2 mg C g⁻¹ soil for the red clay and sandy soils, respectively. The highest SOC content was 6.8 mg C g⁻¹ soil in the sandy soil under MR, whereas for the red clay soil, TR had the highest SOC content of 20.4 mg C g⁻¹ soil. Organic C in the size fractions increased with decreasing size of the fractions. In both soils, the smallest response to management was observed in the clay size fractions, confirming that this size fraction is the most stable. The coarse sand-size fraction was most responsive to management in the sandy soil where MR had 42% more organic C than CR, suggesting that SOC contents of this fraction are predominantly controlled by amounts of C input. In contrast, the fine sand fraction was the most responsive fraction in the red clay soil with a 66% greater C content in the TR than CT. This result suggests that tillage disturbance is the dominant factor reducing C stabilization in a clayey soil, probably by reducing C stabilization within microaggregates. In conclusion, developing viable conservation agriculture practices to optimize SOC contents and long-term agroecosystem sustainability should prioritize the maintenance of C inputs (e.g. residue retention) to coarse textured soils, but should focus on the reduction of SOC decomposition (e.g. through reduced tillage) in fine textured soils.     

Effect of different tillage systems on the quality and crop productivity of a clay–loam soil in semi-arid north-western Turkey

A 2-year study was conducted to investigate the effect of three tillage systems on the properties of clay–loam soil (EutricVertisol) planted with winter wheat (Triticum aestivum L.) in the Canakkale province of north-western Turkey. Crop productivity was also evaluated. The three tillage treatments were: (1) conventional tillage involving mouldboard ploughing followed by two discings (MT); (2) shallow tillage consisting of rototilling followed by one discing (RT); (3) double discing (DD). In the first year of the study, bulk density (BD) was found significantly lower under RT at both 0–10 and 10–20 cm depths with 1.24 and 1.32 Mg cm⁻³, respectively, when compared to MT treatment. However, MT at 20–30 cm provided the highest BD, at 1.49 Mg cm⁻³. In the second year of the study, DD had the lowest BD at all depths followed by RT and MT. Based on the 2-year mean, aggregate size distribution (ASD) and mean weight diameter (MWD) were significantly influenced by tillage treatments. The greatest MWD was obtained with DD, followed by MT and RT. Increasing MWD and coarse aggregates decreased seedling emergence. Organic carbon increased after RT, DD, and MT by 58%, 30%, and 18%, respectively, when compared to the amount at the beginning of the study. Similarly, the total N in the soil and straw was higher after RT than the other treatments. At 1.76 MPa, penetration resistance at 18–30 cm was significantly higher during the growing period using DD, followed by RT with 1.35 MPa and MT with 1.33 MPa. There was no significant difference between treatments at 0–18 cm. Increasing OC and total N and decreasing BD and PR under RT increased grain yield to 4611 kg ha⁻¹, followed by MT and DD at 4375 and 4163 kg ha⁻¹, respectively, according to the 2-year mean.     

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.     

Spatial variation of soil organic carbon concentrations and stable isotopic composition in 1-ha plots of forest and pasture in Costa Rica: implications for the natural abundance technique

The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ¹³C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ¹³C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ¹³C was narrowly constrained in the forest (from −27.96 to −26.09‰) but varied from −15.09 to −28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ¹³C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ¹³C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.     

Nitrogen fertiliser rate affects the frequency of nitrate-dissimilating Pseudomonas spp. in the rhizosphere of Lolium perenne grown under elevated pCO2 (Swiss FACE)

The effect of elevated pCO₂ (60 Pa) on the frequency of nitrate-dissimilating Pseudomonas (NDP) was investigated in the rhizosphere of fertilised Lolium perenne swards in the Swiss Free Air Carbon dioxide Enrichment (FACE) experiment. Numbers of cultivable root-associated Pseudomonas were greater under elevated (60 Pa) than under ambient (36 Pa) pCO₂ in both high and low N-fertilised swards. For both pCO₂ conditions, the NDP frequency decreased with closer root proximity to L. perenne roots in low fertilised swards. Anyway, in high N swards the NDP frequency was similar in root and soil fractions. Thus, N availability may be a major factor influencing NDP populations under elevated pCO₂, most likely due to increased competition for N between plant and nitrate-dissimilating bacteria.     

Effect of water and nitrogen management on aggregate size and carbon enrichment of soil in rice‐wheat cropping system

A study was carried out on a silty clay loam soil (Typic Haplustept) to evaluate the effect of farmyard manure (FYM) vis‐à‐vis fertilizer and irrigation application on the soil organic C content and soil structure. The fertilizer treatments comprised of eight different combinations of N and FYM and three water regimes. The results indicated that the application of FYM and increasing N rate increased soil organic carbon (SOC) content. Addition of FYM also increased the percentage of large sized water stable aggregates (> 5 mm) and reduced the percentage of smaller size aggregates. This was reflected in an increase in the mean weight diameter (MWD) and improved soil structure. The organic carbon content in macroaggregates (> 1 mm) was greater compared to microaggregates, and it declined with decrease in size of microaggregates. This difference in organic C content between macro‐ and microaggregates was more with higher N dose and FYM treated plots. The effect of residual FYM on MWD and organic C content of the soil after wheat harvest was not significant. The effect was less in deeper layers compared to surface layers of the soil. MWD was significantly correlated with the SOC content for the top two layers.     

Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons

Summary Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter.