Predicting soil organic matter stability in agricultural fields through carbon and nitrogen stable isotopes

In order to evaluate the sustainability and efficiency of soil carbon sequestration measures and the impact of different management and environmental factors, information on soil organic matter (SOM) stability and mean residence time (MRT) is required. However, this information on SOM stability and MRT is expensive to determine via radiocarbon dating, precluding a wide spread use of stability measurements in soil science. In this paper, we test an alternative method, first developed by Conen et al. (2008) for undisturbed Alpine grassland systems, using C and N stable isotope ratios in more frequently disturbed agricultural soils. Since only information on carbon and nitrogen concentrations and their stable isotope ratios is required, it is possible to estimate the SOM stability at greatly reduced costs compared to radiocarbon dating. Using four different experimental sites located in various climates and soil types, this research proved the effectiveness of using the C/N ratio and δ¹⁵N signature to determine the stability of mOM (mineral associated organic matter) relative to POM (particulate organic matter) in an intensively managed agro-ecological setting. Combining this approach with δ¹³C measurements allowed discriminating between different management (grassland vs cropland) and land use (till vs no till) systems. With increasing depth the stability of mOM relative to POM increases, but less so under tillage compared to no-till practises. Applying this approach to investigate SOM stability in different soil aggregate fractions, it corroborates the aggregate hierarchy theory as proposed by Six et al. (2004) and Segoli et al. (2013). The organic matter in the occluded micro-aggregate and silt & clay fractions is less degraded than the SOM in the free micro-aggregate and silt & clay fractions. The stable isotope approach can be particularly useful for soils with a history of burning and thus containing old charcoal particles, preventing the use of ¹⁴C to determine the SOM stability.     

Artificial Neural Network estimation of soil erosion and nutrient concentrations in runoff from land application areas

The transport of sediment and nutrients from land application areas is an environmental concern. New methods are needed for estimating soil and nutrient concentrations of runoff from cropland areas on which manure is applied. Artificial Neural Networks (ANNs) trained with a backpropagation (BP) algorithm were used to estimate soil erosion, dissolved P (DP) and NH₄–N concentrations of runoff from a land application site near Lincoln, Nebraska, USA. Simulation results from ANN-derived models showed that the amount of soil eroded is positively correlated with rainfall and runoff. In addition, concentrations of DP and NH₄–N in overland flow were related to measurements of runoff, EC and pH. Coefficient of determination values (R²) relating predicted versus measured estimates of soil erosion, DP, and NH₄–N were 0.62, 0.72 and 0.92, respectively. The ANN models derived from measurements of runoff, electrical conductivity (EC) and pH provided reliable estimates of DP and NH₄–N concentrations in runoff.     

Grain concentrations of protein and mineral nutrients in a large collection of spelt wheat grown under different environments

A large number of spelt wheat genotypes (ranging from 373 to 772) were evaluated for grain concentrations of protein and mineral nutrients under 6 different environments. There was a substantial genotypic variation for the concentration of mineral nutrients in grain and also for the total amount of nutrients per grain (e.g., content). Zinc (Zn) showed the largest genotypic variation both in concentration (ranging from 19 to 145 mg kg⁻¹) and content (ranging from 0.4 to 4.1 μg per grain). The environment effect was the most important source of variation for grain protein concentration (GPC) and for many mineral nutrients, explaining between 37 and 69% of the total sums of squares. Genotype by environment (G × E) interaction accounted for between 17 and 58% of the total variation across the minerals. GPC and sulfur correlated very significantly with iron (Fe) and Zn. Various spelt genotypes have been identified containing very high grain concentrations of Zn (up to 70 mg kg⁻¹), Fe (up to 60 mg kg⁻¹) and protein (up to 30%) and showing high stability across various environments. The results indicated that spelt is a highly promising source of genetic diversity for grain protein and mineral nutrients, particularly for Zn and Fe.     

Influence of biological aggregating agents associated with microbial population on soil aggregate stability

In this study, we investigated the effects of plant residue decomposition and biological aggregating agents (microbial extracellular polysaccharides and fungal hyphae) on soil aggregate stability and determined the microbial population at different stages of soil aggregate stabilization. Experiments were conducted in a 40 days incubation period with the following six treatments: the control (soil only), soil + fungicide, soil + bactericide, soil + maize residues, soil + maize residues + fungicide, and soil + maize residues + bactericide. The maize residues treatments greatly enhanced the formation of macroaggregates. In the residue treatments, the addition of fungicide led to a significant suppression of fungal biomass and activity as well as a reduction of soil aggregate stability, which demonstrated the profound influence of fungal activity on aggregate formation. The addition of bactericide also significantly reduced soil aggregate stability, indicating that bacterial activity also played an important role in the macroaggregate formation. However, the effect of microbial extracellular polysaccharides on soil aggregate stability was not significant, which might be attributable to the fast wet sieving method used for aggregate separation. For the treatments of soil + residues and soil + residues + bactericide, the temporal variations of soil aggregate formation with two peak values suggested that other factors, such as hydrophobic compounds and phenolic acids, might be involved in the soil aggregate stabilization process.     

Water availability and abundance of microbial groups are key determinants of greenhouse gas fluxes in a dryland forest ecosystem

Forests are considered key biomes that could contribute to minimising global warming as they sequester carbon (C) and contribute to mitigate emissions of the potent greenhouse gases (GHG) including nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂). Management practices are prevalent in forestry, particularly in dryland ecosystems, known to be water and nitrogen (N) limited. Irrigation and fertilisation are thus routinely applied to increase the yield of forest products. However, the contribution of forest management practices to current GHG budgets and consequently to soil net global warming potential (GWP) is still largely unaccounted for, particularly in dryland ecosystems. We quantified the long-term effect (six years) of irrigation and fertilisation and the impact of land-use change, from grassland to a Eucalyptus plantation on N₂O, CH₄ and CO₂ emissions and soil net GWP, within a dryland ecosystem. To identify biotic and abiotic drivers of GHG emissions, we explored the relationship of N₂O, CH₄ and CO₂ fluxes with soil abiotic characteristics and abundance of ammonia-oxidizers, N₂O-reducing bacteria, methanotrophs and total soil bacteria. Our results show that GHG emissions, particularly N₂O and CO₂ are constrained by water availability and both N₂O and CH₄ are constrained by N availability in the soil. We also provide evidence of functional microbial groups being key players in driving GHG emissions. Our findings illustrate that GHG emission budgets can be affected by forest management practices and provide a better mechanistic understanding for future mitigation options.     

不同水分下水铁矿在土壤中稳定性变化对砷移动性的影响

为合理利用水铁矿,将其作为土壤砷污染修复的一种高效稳定的钝化剂,采集湖南石门县三个不同乡镇的土壤进行室内培养试验,采用连续提取法和薄膜梯度扩散技术(D iffusive gradient in the thin films technique,DGT),研究不同水分模式下外源添加的水铁矿在土壤中稳定性变化过程中对砷在土壤中移动性的影响。结果表明,水铁矿在三种不同水分管理模式[100%SWHC(Soilwaterholdingcapacity,SWHC)、干湿交替(D ry/wetcycle,DWC)和30%SWHC]下培养33d后均发生转化,其中在100%SWHC条件下转化速率最快,在三种土壤中的转化率达到16.50%~22.78%,在30%SWHC和DWC条件下最大转化率仅为8.77%和10.96%。添加外源水铁矿后在三种水分管理模式下两种砷污染土壤中活性态砷浓度均显著降低,并且在100%SWHC条件下降幅最大,在中高浓度的砷污染土壤中降低程度分别达到42.08%和60.75%。但随后在100%SWHC和DWC条件下培养22d后,活性态砷的浓度均出现微弱的上升趋势。在100%SWHC和DWC两种水分条件下,两种砷污染土壤中弱结晶态铁氧化物结合态砷(F1-As)的含量均显著增加,并且随着水铁矿的转化而逐渐向更稳定的形态(F2-As)迁移,在30%SWHC条件下F2-As含量没有增加。研究表明,水铁矿在土壤高水分条件下稳定性最差,其次为土壤的干湿交替条件。在两种砷污染土壤中施加水铁矿修复过程中都具有潜在的砷释放风险,在农业生产中应合理调控土壤水分以保证水铁矿对砷的吸附效率。     

Effect of depth of fertilizer banded-placement on growth,nutrient uptake and yield of oilseed rape (Brassica napus L.)

A better understanding of crop growth and nutrient uptake responses to the depth of fertilizer banded-placement in the soil is needed if growth and nutrient uptake responses are to be maximized. A two-year field study covering two rape seasons (2010–2011 and 2011–2012) was conducted to examine the effect of banded-placement of N–P–K fertilizer at various depths on growth, nutrient uptake and yield of oilseed rape (Brassica napus L.). The results showed that fertilization at 10 cm and 15 cm soil depth produced greater taproot length and dry weight than fertilization at 0 cm and 5 cm. 0 cm and 5 cm deep fertilization significantly increased the lateral root distribution at 0–5 cm soil depth, while 10 cm and 15 cm deep fertilization induced more lateral root proliferation at 5–15 cm soil depth. At 36 days after sowing (DAS), 5 cm deep fertilization produced better aboveground growth and nutrient uptake than 10 cm and 15 cm deep fertilization. However, reversed results were observed after 36 DAS. 10 cm and 15 cm deep fertilization produced more rapeseed than 0 cm and 5 cm deep fertilization, moreover, the yield difference was more significant in drought season (2010–2011) than in relatively normal season (2011–2012). In summary, these results preliminarily suggest that both 10 cm and 15 cm are relatively proper fertilizer placement depth when the practice of banding fertilizer is used in oilseed rape production. But from the viewpoint of diminishing the production cost, 10 cm deep fertilization should be recommended in actual farming. Because 15 cm deep fertilization may require higher mechanical power input, and thus resulting in higher cost of production.     

中国玉米秸秆草谷比及其资源时空分布特征

针对玉米秸秆资源量及时空区域分布不清等问题，该研究分析9个典型省的玉米秸秆草谷比差异性，并基于草谷比实测值，评价近10a中国玉米秸秆资源量的时空变化情况，预测玉米秸秆的资源潜力。研究结果表明，玉米秸秆草谷比实测值为（0.84±0.23），不同地区、不同品种草谷比差异显著，随着年份变化，玉米品种和种植方式在不断变化，草谷比逐年变小，从2009年1.2减小到2018年的0.84，估算2018年全国玉米秸秆理论资源量为2.16×108 t，比2009年仅增加3.9%。玉米秸秆东北和华北地区资源量最高，占50%以上，与2009年相比，东北、华北、西北地区资源量有所增加，华东、华中、西南、华南略有下降；单位面积玉米秸秆可收集资源量4.51 t/ha，比2009年增加23%，东北地区最高，其次华北、华东和西北地区，然后是华中和西南地区，华南地区最低。预测2025年玉米秸秆的理论资源量为(2.53±0.58)×108 t，可收集资源量为(1.86±0.51)×108 t。研究为全国各个地区的秸秆合理规划利用提供基本参考数据。     

Winter wheat canopy interception and its influence factors under sprinkler irrigation

Field studies on winter wheat canopy interception with its relations to leaf area index (LAI), plant height, drop diameter, wind speed, and water application intensity were carried out. Canopy interception was measured using the water wiping method. Results indicate that the maximum value of winter wheat canopy interception was not more than 1.0 mm, much smaller than presented by previous investigators. The total canopy interception for the growing season was 2.4 mm, only 1.3% of the total irrigation amount (194.6 mm), for four sprinkler irrigation events during 2003. Canopy interception increased as leaf area index and plant height increased. A linear regression model was developed to express the relationship of canopy interception with leaf area index and plant height. There was good agreement between the values of canopy interception measured using the water wiping method and estimated using the linear regression model. Results also indicate that canopy interception decreased as drop diameter and wind speed increased. An exponential relationship was found between canopy interception and drop size, and a linear relationship between canopy interception and the square of the wind speed. Water application intensity does not affect canopy interception significantly.     

Nitrous oxide emissions following seasonal freeze-thaw events from arable soils in Northeast China

Seasonal soil freeze-thaw events may enhance soil nitrogen transformation and thus stimulate nitrous oxide(N_2O)emissions in cold regions.However,the mechanisms of soil N_2O emission during the freeze-thaw cycling in the field remain unclear.We evaluated N_2O emissions and soil biotic and abiotic factors in maize and paddy fields over 20 months in Northeast China,and the structural equation model(SEM)was used to determine which factors affected N_2O production during non-growing season.Our results verified that the seasonal freeze-thaw cycles mitigated the available soil nitrogen and carbon limitation during spring thawing period,but simultaneously increased the gaseous N_2O-N losses at the annual time scale under field condition.The N_2O-N cumulative losses during the non-growing season amounted to 0.71 and 0.55 kg N ha~(–1) for the paddy and maize fields,respectively,and contributed to 66 and 18%of the annual total.The highest emission rates(199.2–257.4μg m~(–2) h~(–1))were observed during soil thawing for both fields,but we did not observe an emission peak during soil freezing in early winter.Although the pulses of N_2O emission in spring were short-lived(18 d),it resulted in approximately80%of the non-growing season N_2O-N loss.The N_2O burst during the spring thawing was triggered by the combined impact of high soil moisture,flush available nitrogen and carbon,and rapid recovery of microbial biomass.SEM analysis indicated that the soil moisture,available substrates including NH_4~+and dissolved organic carbon(DOC),and microbial biomass nitrogen(MBN)explained 32,36,16 and 51%of the N_2O flux variation,respectively,during the non-growing season.Our results suggested that N_2O emission during the spring thawing make a vital contribution of the annual nitrogen budget,and the vast seasonally frozen and snow-covered croplands will have high potential to exert a positive feedback on climate change considering the sensitive response of nitrogen biogeochemical cycling to the freeze-thaw disturbance.