1. The legacy of cropping history reduces the recovery of soil carbon and nitrogen after conversion from continuous cropping to permanent pasture

• Authors:
  • Orton, T. G.
  • Jones, A. R.
  • Dalal, R. C.
• Source: Science Article
• Volume: 216
• Year: 2016
• Summary: Enhancing soil organic carbon (SOC) and total nitrogen (N) is considered an important step in mitigating greenhouse gas emissions and improving soil fertility. The loss of SOC and N generally observed during cropping may be reversed by converting such land use to permanent pasture. However, large uncertainties remain around the processes that govern how much C and N may be sequestered from this conversion in soils worldwide. Here, we sampled soils across 10 paddocks on 20-year old grass pasture sites with a chronosequence of cropping history in order to quantify changes in SOC and N after the conversion of long-term cropping to pasture land use in a semi-arid region of southwest Queensland, Australia. The average rate of change in SOC stocks under pasture in the top 0-0.1m soil layer was approximately 0.1tCha-1yr-1, consisting of an increase in SOCC4 (pasture) of 0.2tCha-1yr-1 and a decrease in SOCC3 (pre-pasture) of 0.1tCha-1yr-1. The decrease in SOCC3 was enhanced at sites with greater years under cropping, indicating a reduced potential for SOC sequestration at sites with longer duration under cropping. The loss of total nitrogen (N) under cropping was also partially recovered with the introduction of permanent perennial pastures. A significant, positive correlation between soil aggregation and mineralisable N under cropping suggested that soil structure has a strong influence over N stability in the soil. However, soil aggregation and mineralisable N did not improve under pastures, indicating that the loss of soil fertility and structure under cropping remained a residual effect that was not recovered within 20 years of permanent pastures in this semi-arid subtropical environment. We suggest here that the resilience of ecosystems to recover soil fertility under pastures declines with greater years under cropping. © 2015 Elsevier B.V.

2. Effects of straw return and regional factors on spatio-temporal variability of soil organic matter in a high-yielding area of northern China

• Authors:
  • Hu, K.
  • Wei, Y.
  • Wu, W.
  • Zheng, L.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Understanding the effects of straw return and regional factors on changes of soil organic matter (SOM) is helpful for improving soil quality and sustaining high crop productivity in intensive agro-ecosystems. This study focuses on the changes in SOM of agricultural soils in a high yielding area of Huantai County, northern China, for which data are available for 1982, 1996 and 2011. The spatio-temporal patterns of SOM, and the factors that influence them, were analyzed before and after widespread implementation of straw retention practices in large areas. The results showed that the average SOM contents in the cultivated layer were 13.24, 15.06 and 18.73gkg-1 in 1982, 1996 and 2011, respectively. The rate of increase in SOM contents during 1996-2011 was approximately twice that of 1982-1996. Semivariogram analysis results showed that the spatial correlation distances of SOM in 1982 and 1996 were 8.78km and 4.02km, respectively, while the pattern was a pure nugget effect in 2011, reflecting that the influence of human activities on SOM content increased over time. During 1982-1996 straw retention practices had not been implemented in large areas and the increase of average SOM content was relatively small, and even declined in some sub-regions. In contrast, the average SOM content of the entire county generally increased from 1996 to 2011 after straw return was implemented in large areas. The effects of topography on SOM distribution can be ignored because the county is relatively flat. From 1982 to 1996 regional factors such as soil type and soil texture had moderate impacts on SOM distribution. From 1996 to 2011 increased straw return resulted in substantially increased SOM, while the effects of soil type and soil texture were weakened. Straw return increased carbon sequestration in an intensive agro-ecosystem in this region and should be recommended as a long-term management practice to improve soil fertility and sustain high crop yields, as well as to store carbon and reduce greenhouse gas emissions. © 2014 Elsevier B.V.

3. Long-term effect of sediment laden Yellow River irrigation water on soil organic carbon stocks in Ningxia, China

• Authors:
  • Shi, X.
  • Liu, Y.
  • Jin, W.
  • Zhang, M.
  • Zhang, H.
  • Yu, D.
  • Dong, L.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Given the positive effects of mediating the growth of greenhouse gases in the atmosphere, interest in soil carbon stock dynamics has greatly increased. Several questions still exist as to whether irrigation with sediment laden water benefits carbon sequestration in soil profiles. This case study documented how long-term irrigation with sediment laden water from the Yellow River affected soil carbon sequestration in the Ningxia Irrigation Zone, China. The study included eight durations of irrigation management (10, 20, 30, 50, 280, 1300, 2100, and 2200 years) and five soil types. Soil samples from 44 profiles were collected to a depth of 100. cm, divided into four layers (0-20, 20-30, 30-60, 60-100 cm), and analyzed for soil organic carbon (SOC). SOC stocks both of soil profiles (0-100 cm) and irrigation-silted soil (ISS) layer, were 28.2 Tg C and 24.1 Tg C, respectively. The ISS layer was formed by the overlapping actions of irrigation and tillage, manure addition, and sediment silting as a result of long-term irrigation from the sediment laden water of the Yellow River. Compared to non-irrigated and non-cultivated control soils of similar depths and thicknesses, SOC stocks of the ISS layer increased 16.9. Tg C, and accounted for 89.9% of a total increment of 18.8 Tg C in the 0-100 cm layer of irrigated cropland soils. A significant correlation was found between the SOC density increment of the ISS layer and the number of irrigation years. Long-term irrigation with sediment laden Yellow River water greatly influenced SOC stocks, especially in the ISS layer, which plays an important role in soil carbon sequestration.

4. Conservation tillage of rainfed maize in semi-arid Zimbabwe: A review

• Authors:
  • Stroosnijder, L.
  • Nyakudya, I. W.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Food security in Sub-Saharan Africa, particularly in semi-arid tropics (41% of the region; 6 months of dry season) is threatened by droughts, dry spells and infertile soils. In Zimbabwe, 74% of smallholder farming areas are located in semi-arid areas mostly in areas with soils of low fertility and water holding capacity. The dominant crop in these areas, maize (Zea mays L.), is susceptible to drought. Under smallholder farming in Zimbabwe, conventional tillage entails cutting and turning the soil with a mouldboard plough thereby burying weeds and crop residues. Seed is planted by hand into a furrow made by the plough, ensuring that crops germinate in relatively weed free seedbeds. Inter-row weed control is performed using the plough or ox-drawn cultivators and hand hoes. Conventional tillage has been criticised for failure to alleviate negative effects of long dry spells on crops and to combat soil loss caused by water erosion estimated at 50 to 80tha-1yr-1. Therefore, conservation tillage has been explored for improving soil and water conservation and crop yields. Our objective was to determine the maize yield advantage of the introduced technology (conservation tillage) over conventional tillage (farmers' practice) based on a review of experiments in semi-arid Zimbabwe. We use a broad definition of conservation tillage instead of the common definition of =30% cover after planting. Eight tillage experiments conducted between 1984 and 2008 were evaluated. Conventional tillage included ploughing using the mouldboard plough and digging using a hand hoe. Conservation tillage included tied ridging (furrow diking), mulch ripping, clean ripping and planting pits. Field-edge methods included bench terraces (fanya juus) and infiltration pits. Results showed small yield advantages of conservation tillage methods below 500mm rainfall. For grain yields =2.5tha-1 and rainfall =500mm, 1.0m tied ridging produced 144kgha-1 and mulch ripping 344kgha-1 more than conventional tillage. Above 2.5tha-1 and for rainfall >500mm, conventional tillage had =640kgha-1 yield advantage. Planting pits had similar performance to ripping and conventional tillage but faced digging labour constraints. Experiments and modelling are required to test conservation tillage seasonal rainfall thresholds. Constraints to adoption of conservation tillage by smallholder farmers necessitate best agronomic practices under conventional tillage while work on adoption of alternative tillage methods continues.

5. Estimating residual water content in air-dried soil from organic carbon and clay content

• Authors:
  • Eriksson, J.
  • Poeplau, C.
  • Kätterer, T.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Measurement of soil organic carbon (SOC) or other elements in air-dried soil samples leads to underestimation of mass fraction per unit dry soil (concentration) unless the residual water (RW) content is accounted for. However, RW measurements are time-consuming and costly and thus often neglected. The resulting bias can lead to dramatic erroneous results, especially when two differently treated datasets are compared. We therefore, derived pedotransfer functions to estimate residual water content in air-dried soil samples from: (i) SOC content, (ii) clay content and (iii) both variables together. The uncertainty of prediction decreased in the order (i)-(iii), with root mean squared deviation (RMSD) values of 0.64, 0.46 and 0.34% of RW content, respectively. These functions represent a potential step towards more harmonized and transparent SOC determination.

6. Towards on-the-go field assessment of soil organic carbon using Vis-NIR diffuse reflectance spectroscopy: Developing and testing a novel tractor-driven measuring chamber

• Authors:
  • Amelung, W.
  • Berg, T.
  • Damerow, L.
  • Welp, G.
  • Rodionov, A.
  • Pätzold, S.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Proximal sensing of soil organic carbon (SOC) in the field using diffuse reflectance spectroscopy is still difficult under variable weather conditions. Here, we introduce a tractor-driven measuring chamber for on-the-go visible and near-infrared diffuse reflectance spectroscopy (Vis-NIRS) meeting experimental and precision agriculture demands. A commercial full-range spectrometer operates in a closed dark chamber with artificial light. Sensor view angle, distance to soil, and illumination conditions were optimized. The mobile chamber was placed on drum rollers to flatten the ploughed and tilled soil surface and to minimize disturbances in Vis-NIR spectra by surface roughness. Prior to on-the-go spectra acquisition under field conditions, SOC prediction models for the soils under study were independently calibrated under variable moisture and roughness conditions. Driving at a tractor velocity of 3kmh-1 resulted in measuring spots of approximately 8cm length and 3cm width at 0.6m distance to one another in the direction of movement, delivering geo-referenced SOC concentrations at a sub-m spatial resolution. Gravel on the soil surface resulted in erratic extremes of predicted SOC concentrations, but these could be eliminated as outliers. The system was tested under field conditions on two long-term experiments at two different sites which revealed each a large span of SOC concentrations. On-the-go predicted SOC concentrations and those obtained from conventional plot-wise lab analyses were correlated with coefficients of determination of R2=0.65 and a standard error of 1.22g SOCkg-1. Further improvements, particularly in data processing, will enable a reliable proximal sensing on-the-go for precision agriculture purposes in the near future.

7. Effects of plastic mulch and crop rotation on soil physical properties in rain-fed vegetable production in the mid-Yunnan plateau, China

• Authors:
  • Zhang, X. X.
  • Hu, X. B.
  • Zhang, G. S.
  • Li, J.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Plastic mulch is widely used for vegetable cropping in the mid-Yunnan plateau, southwestern China. However, the effects of plastic mulch on soil physical properties are poorly understood. A field experiment was conducted to evaluate the effects of different plastic mulch patterns (narrow and wide plastic mulch) and rotations (broccoli-zucchini-winter wheat and broccoli-zucchini-fallow) on soil physical properties and soil organic carbon content in a vegetable production system at a research station in the region. The experiment comprised four treatments and laid out in the field using randomized complete block design replicated nine times. The soil (0-20cm) under wide plastic mulch retained more water than the soil narrow plastic mulch under the vegetable growing season over 3 consecutive severe drought years. Significant decline (11%) of surface soil (0-5cm) organic carbon was observed in 2012 compared with in 2010, but soil organic carbon and physical properties did not differ significantly between the two different plastic mulch patterns. The higher macro-porosity, aggregate stability, Ksat and lower bulk density in the plastic mulched ridges imply that the conversion from narrow-plastic-mulch to wide-plastic-mulch increases mulched area thereby conserving larger area soil structure in the croplands. Although increased catch crop stubble retention in the furrows apparently improved surface soil macro-porosity and saturated hydraulic conductivity, the autumn rotary cultivation in broccoli-zucchini-winter wheat rotation system has significantly decreased (5%) surface soil organic carbon. The results suggested that continued use of vegetable-cereal crop rotation system, even with stubble retention, may result in loss soil organic carbon. Further research that covers wetter years should be taken to assess effects of vegetable-cereal crop rotation pattern on soil physical properties in this region.

8. Overall assessment of soil quality on humid sandy loams: Effects of location, rotation and tillage

• Authors:
  • Munkholm, L. J.
  • Rickson, R. J.
  • Hansen, E. M.
  • Abdollahi, L.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 145
• Year: 2015
• Summary: Conservation tillage and diversified crop rotations have been suggested as appropriate alternative soil management systems to sustain soil quality. The purpose of this study was to quantify the effect of implementing three crop rotations (R2-R4) on soil structural changes and the "productivity function" of soil. R2 is a winter-dominated crop rotation (winter wheat was the main crop) with straw residues incorporated. R3 is a mix of winter and spring crops with straw residues removed. R4 is the same mix of crops as in R3, but with straw residues incorporated. Three tillage systems were used for each rotation: mouldboard ploughing to a depth of 20. cm (MP); harrowing to a depth of 8-10. cm (H); and direct drilling (D) at two experimental sites with a sandy loam soil and different water budgets in Denmark. The Muencheberg soil quality rating (M-SQR) method and simpler soil quality indices (i.e. visual evaluation of soil structure (VESS), overall visual structure (OVS) and overall soil structure (OSS)) were employed to differentiate the effects of these alternative management practices on soil structural quality and relative crop yield (RY). A Pearson correlation was also employed to find the correlation between the soil quality indices and relative crop yield. Relevant soil properties for calculating the soil quality indices were measured or obtained from previous publications. Crop rotation affected the soil structure and RY. The winter-dominated crop rotation (R2) resulted in the poorest soil structural quality and produced the lowest RY compared to the mixed rotations (R3 and R4). Tillage systems clearly influenced the soil quality and RY. The MP resulted in the best soil structural quality, and consequently the highest RY compared with both the reduced tillage treatments. Significant correlations were found in most cases between soil quality indices (including M-SQR) and RY. This highlights the influence of soil quality (as measured by the selected indicators) - and soil structure in particular - on crop yield potential. © 2014 Elsevier B.V.

9. Carbon Sequestration by Perennial Energy Crops: Is the Jury Still Out?

• Authors:
  • Richter, G. M.
  • Gregory, A. S.
  • Agostini, F.
• Source: BioEnergy Research
• Year: 2015
• Summary: Soil organic carbon (SOC) changes associated with land conversion to energy crops are central to the debate on bioenergy and their potential carbon neutrality. Here, the experimental evidence on SOC under perennial energy crops (PECs) is synthesised to parameterise a whole systems model and to identify uncertainties and knowledge gaps determining PECs being a sink or source of greenhouse gas (GHG). For Miscanthus and willow (Salix spp.) and their analogues (switchgrass, poplar), we examine carbon (C) allocation to above- and belowground residue inputs, turnover rates and retention in the soil. A meta-analysis showed that studies on dry matter partitioning and C inputs to soils are plentiful, whilst data on turnover are rare and rely on few isotopic C tracer studies. Comprehensive studies on SOC dynamics and GHG emissions under PECs are limited and subsoil processes and C losses through leaching remain unknown. Data showed dynamic changes of gross C inputs and SOC stocks depending on stand age. C inputs and turnover can now be specifically parameterised in whole PEC system models, whilst dependencies on soil texture, moisture and temperature remain empirical. In conclusion, the annual net SOC storage change exceeds the minimum mitigation requirement (0.25 Mg C ha−1 year−1) under herbaceous and woody perennials by far (1.14 to 1.88 and 0.63 to 0.72 Mg C ha−1 year−1, respectively). However, long-term time series of field data are needed to verify sustainable SOC enrichment, as the physical and chemical stabilities of SOC pools remain uncertain, although they are essential in defining the sustainability of C sequestration (half-life >25 years).

10. Long-term soil organic carbon and nitrogen dynamics after conversion of tropical forest to traditional sugarcane agriculture in East Mexico

• Authors:
  • Huber-Sannwald, E.
  • Anaya, C. A.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 147
• Year: 2015
• Summary: Conversion of tropical forests to croplands greatly reduces stock size of soil organic resources. Low-impact agriculture may curtail this loss in soil fertility, and favor long-term agroecosystem functioning. In east tropical Mexico, indigenous smallholders have been managing sugarcane plots without burning, fertilizing, tillaging, and removal of crop residues for the last 70 years. This study examined the long-term effects of such low-input sugarcane agriculture on soil aggregate structure and soil organic carbon (SOC) and soil nitrogen (N) stocks along a 50-year chronosequence of cultivation including differently aged sugarcane plots and tropical forests as a reference site. Soils were physically fractioned in four aggregate size classes and in four soil organic matter (SOM) fractions and changes in C concentrations and C/N ratios of these fractions were assessed. Long-term cultivation did not alter soil aggregate structure, or litter C content; however it reduced significantly litter N content in sugarcane compared to forest plots. After 20 years of cultivation, SOC and soil N stocks dropped by 25% at 0-10. cm soil depth compared to current C stocks in forest soils. After 50 years of cultivation, pooled over 0-20. cm, SOC stocks were similar in sugarcane and forest plots, while soil N stocks remained 12-19% lower in sugarcane than in forest plots. The mineral-associated organic C fraction remained unaffected by land use change. Forest conversion to sugarcane, depleted the free light particulate organic matter (POM) C fraction at 0-10. cm depth. Forest intra-aggregate POM C concentration declined after 20 years of cultivation and then recovered after 50 years of cultivation at 0-10. cm depth. This study demonstrates that traditional sugarcane cropping may be a long-term sustainable alternative to sequester SOC, while maintaining moderate soil N fertility levels without trading off on sugarcane production.