基于土壤微波辐射布儒斯特角反演土壤含水率

在利用被动微波遥感技术进行裸露地表土壤含水率(Soil Moisture Content,SMC)的反演中,土壤粗糙度是制约反演精度的最关键因素。该研究利用改进的积分方程模型(Advanced Integral Equation Model,AIEM)进行地表多角度微波发射率的模拟,探索地表微波辐射多角度信息用于提高地表SMC反演精度的可行性。基于不同SMC和不同粗糙度地表多角度V极化发射率数据的变化趋势提取土壤介质布儒斯特角,结果表明,土壤布儒斯特角对SMC具有较高的敏感性,C波段(6.6 GHz)不同含水率土壤的布儒斯特角分布在60°～80°范围内。基于AIEM模拟数据的分析发现,土壤布儒斯特角正切值与SMC具有较好的线性关系,线性拟合决定系数为0.94,均方根误差为0.027cm~3/cm~3,并得到了基于布儒斯特角的裸露地表SMC反演算法。基于模拟数据的算法验证结果表明,算法的SMC预测值与理论值的决定系数为0.95,均方根误差为0.024 cm~3/cm~3。算法在不同土壤粗糙度自相关函数下均表现出稳健的特性,SMC预测精度最大均方根误差为0.027 cm~3/cm~3,最小为0.023cm~3/cm~3。基于布儒斯特角的SMC反演算法利用的是多角度土壤发射率的相对变化而非其绝对数值,该研究为SMC的多角度被动微波遥感提供了一种不同的研究思路。     

水盐交互作用对河套灌区土壤光谱特征的影响

为探究土壤水盐交互作用对Sentinel-2卫星光谱特征的影响，该研究以内蒙古河套灌区沙壕渠灌域为研究区域，分别在2018年和2019年的4-5月共采集280个裸土期表层土壤样本，测定其土壤含水率和含盐量，并获取同步的Sentinel-2卫星遥感数据，构建基于土壤水盐-反射率原理的土壤光谱特征理论模型，在此基础上结合土壤水盐交互作用构建水盐交互模型，并比较2种模型对土壤光谱的模拟效果，分析土壤水盐交互作用对土壤光谱估算的影响。结果表明：1）土壤水盐交互作用对光谱的影响因波段类型和水盐含量的不同而有所不同。在可见光范围上影响相对较弱，其作用范围为-0.11～0.29；在近红外和短波红外范围上影响相对较强，其作用范围为-0.35～0.61；当水分或盐分中某个含量较高时对光谱影响较弱，其主要集中在-0.1～0.23；，在水盐含量程度相似时影响较强，其作用范围为0.3～0.6。2）与土壤光谱特征理论模型相比，水盐交互模型能明显地改善土壤光谱的模拟效果，能将模拟相关系数由0.14～0.44提升到0.29～0.70，均方根误差由0.032～0.082降低到0.029～0.068。该研究结果揭示了盐分和水分对光谱特征的干扰过程，为土壤盐分的估算提供策略与方法，对实现区域尺度上土壤盐分的精准监测具有一定的意义。     

土壤微生物膜对风沙土固沙保水特性的影响

土壤微生物膜胞外聚合物具有较强的黏结性,为增强风沙土抗蚀性,提高风沙土保水性提供了新途径。鉴于此,该研究采用室内控制试验设置6个微生物菌剂施用水平(0、1、3、5、7和10 g/kg),以期阐明土壤微生物膜对风沙土特性的影响。研究结果表明:1)施用微生物菌剂可产生土壤微生物膜,并胶结风沙土颗粒。2)试验结束时(第49 d),与对照组相比,不同菌剂施用处理的土壤容重降低0.54%～8.88%,孔隙度提高0.39%～3.91%,含水率显著提高0.11%～0.25%(P0.05),大团聚体质量分数增加5.59%～25.01%,土壤pH值由7.16显著增加至8.32～9.23(P0.05)。3)土壤多糖含量对土壤特性的解释率为47%,是影响土壤特性的关键因素。土壤微生物膜能够有效改善风沙土特性,增强风沙土抗蚀性,提高风沙土保水性,但较高的微生物菌剂施用量会增加土壤pH值,造成土壤盐碱化,建议微生物菌剂施用水平为1～5 g/kg。研究成果有利于深入理解微生物膜固沙保水机制,并可为沙化土地防治新技术研发提供重要理论依据和科技支撑。     

氧化石墨烯和五价砷在改性多孔介质中的共迁移特征

砷是农田土壤重金属污染的主要元素之一,在砷污染农田土壤的修复过程中往往忽视纳米颗粒能够使结合态的砷重新释放,导致有效态砷浓度升高,探究土壤中黏土矿物对氧化石墨烯(Graphene Oxide,GO)和五价砷(As(V))在多孔介质中迁移行为的影响,对进一步完善农田土壤砷修复理论以及提高农作物产量、保护人体健康具有重要意义。该研究利用蒙脱石和高岭石改性石英砂,通过砂柱迁移试验系统地研究了GO、As(V)和GO-As(V)在填加0%、10%、30%和50%的蒙脱石和高岭石改性石英砂柱中的迁移行为。研究结果表明,随着高岭石和蒙脱石改性石英砂填加比例的增加,GO和As(V)的迁移能力均呈降低趋势,且GO和As(V)在不同条件下的迁移曲线均存在显著差异(P0.05);GO在50%高岭石和蒙脱石改性石英砂柱中的回收率相对于石英砂柱分别下降了14%和17%,As(V)分别下降了15%和12%;在共迁移试验中,GO和As(V)在石英砂柱中回收率分别上升至99%和100%。分析表明,As(V)在蒙脱石改性石英砂柱中的迁移能力大于高岭石改性石英砂,而GO与之相反;当GO和与As(V)共迁移时,二者在介质中的迁移能力均大于其单独迁移。本研究表明GO、As(V)释放到土壤后,能够加速As(V)的迁移,造成土壤砷污染的扩大化。     

基于变量优选与机器学习的干旱区湿地土壤盐渍化数字制图

土壤盐渍化是导致土壤退化和生态系统恶化的主要原因之一，对干旱区的可持续发展构成主要威胁。为了尽可能精确地监测土壤盐渍化的空间变异性，该研究收集新疆艾比湖湿地78个典型样点，其中选取54个样本作为训练集，24个样本作为独立验证集。基于Sientinel-2 多光谱传感器（Multi-Spectral Instrument，MSI）、数字高程模型（Digital Elevation Model，DEM）数据提取3类指数（红边光谱指数、植被指数和地形指数），经过极端梯度提升（Extreme Gradient Boosting，XGBoost）算法筛选有效特征变量，构建了关于土壤电导率（Electrical Conductivity，EC）的随机森林（Random Forest，RF）、极限学习机（Extra Learning Machine，ELM）和偏最小二乘回归（Partial Least Squares Regression，PLSR）预测模型，并选择最优模型绘制了艾比湖湿地盐渍化分布图。结果表明：优选的红边光谱指数基本能够预测EC的空间变化；红边光谱指数与植被指数组合建模效果总体上优于其与地形指数的组合，3类指数组合的建模取得了较为理想的预测精度，其中RF模型表现最优（验证集R2=0.83，RMSE=4.81 dS/m，RPD=3.11）；在整个研究区内，中部和东部地区土壤盐渍化程度尤为严重。因此，XGBoost所筛选出的环境因子结合机器学习算法可以实现干旱区土壤盐渍化的监测。     

Mapping soil salinity and pH across an estuarine and alluvial plain using electromagnetic and digital elevation model data

Increasing pressures from agriculture and urbanization have resulted in drainage of many floodplains along the eastern Australian coastline, which are underlain by sulphidic sediments, to lower water tables and reduce soil salinity. This leads to oxidation of the sediments with a rapid decline in pH and an increase in salinity. Accurately mapping soil salinity and pH in coastal acid sulphate soil (CASS) landscapes is therefore important. One required map is the extent of highly acidic (i.e. pH < 4.5) areas, so that the application of alkaline amendments (e.g. lime) to neutralize the acid produced can be specifically targeted to the variation in pH. One approach is to use digital soil mapping (DSM) using ancillary information, such as an EM38, digital elevation models (DEM – elevation) and trend surface parameters (east and north). We used an EM38 in the horizontal (EM38h) and vertical (EM38v) modes together with elevation data to develop multiple linear regressions (MLR) for predicting EC_1:5 and pH. For pH, best results were achieved when the EM38 EC_a data were log‐transformed. By comparing MLR models using REML analysis, we found that using all ancillary data was optimal for mapping EC_1:5, whereas the best predictors for pH were north, log‐EM38v and elevation. Using residual maximum likelihood (REML), the final EC_1:5 and pH maps produced were consistent with previously defined soil landscape units, particularly CASS. The DSM approach used is amenable for mapping saline soils and identifying areas requiring the application of lime to manage acidic soil conditions in CASS landscape.     

Response of ammonia oxidizing microbes to the stresses of arsenic and copper in two acidic alfisols

Soil pollution by elevated heavy metals exhibits adverse effects on soil microorganisms. Ammonia oxidizing bacteria and ammonia oxidizing archaea perform ammonia oxidative processes in acidic soils. However, influence of heavy metal stress on soil ammonia oxidizers distribution and diversity is inadequately addressed. This study investigated the responses of ammonia oxidizing bacteria and archaea to heavy metals, Cu and As during short-term laboratory experiment. Two different acidic alfisols named as Rayka and Hangzhou spiked with different concentrations of As, Cu and As + Cu were incubated for 10 weeks. Significant reduction in copy numbers of archaeal-16S rRNA, bacterial-16S rRNA and functional amoA genes was observed along elevated heavy metal concentrations. Ammonia oxidizing archaea was found to be more abundant than ammonia oxidizing bacteria in all the heavy metal treatments. The potential nitrification rate significantly decreased with increasing As and Cu concentrations in the two soils examined. Denaturing gradient gel electrophoresis analysis revealed no apparent community shift for ammonia oxidizing archaea even at higher concentrations of As and Cu. Phylogenetic analysis of archaeal amoA gene from 4 clone libraries indicated that all the archaeal amoA sequences were placed within 3 distinct clusters from soil and sediment group 1.1b of Thaumarchaeota. Our results could be useful for the better understanding of the ecological effects of heavy metals on the abundance and diversity of soil ammonia oxidizers.     

Assessing the effectiveness of manure export plus intensive silage cropping for lowering the Olsen‐P status of P‐enriched grassland

A substantial proportion of farmed grassland soils in Northern Ireland (NI) are overly enriched with P and pose a risk to water quality. To address this problem, manure could be exported rather than recycled to P‐enriched land and the latter intensively cropped with grass silage to deplete soil P. To assess the efficacy of such a strategy, a P‐ and K‐enriched grassland site was intensively cropped over a 6‐yr period with fertilizer N alone supplied to support silage growth. By year 6, soil P had declined from index 5 to index 3, and it was estimated that two more years of this management may bring it into the target index 2 range. Soil K, however, declined rapidly from index 4 to index 1 in just 4 yr, with the result that grass production became limited by K deficiency. It was concluded that nonrecycling of manure to P‐enriched grassland under silage management is probably the most effective strategy for lowering soil P status, but care must be taken to prevent K deficiency occurring.     

Oxygen isotope ratios of plant available phosphate in lowland tropical forest soils

Phosphorus (P) cycles rapidly in lowland tropical forest soils, but the process have been proven difficult to quantify. Recently it was demonstrated that valuable data on soil P transformations can be derived from the natural abundance of stable oxygen isotopes in phosphate (δ¹⁸O_P). Here, we measured the δ¹⁸O_P of soils that had received long-term nutrient additions (P, nitrogen, and potassium) or litter manipulations in lowland tropical forest in Panama and performed controlled incubations of fresh soils amended with a single pulse of P. To detect whether δ¹⁸O_P values measured in the incubations apply also for soils in the field, we examined the δ¹⁸O_P values after rewetting dry soils. In the incubations, resin-P δ¹⁸O_P values converged to ∼3.5‰ above the expected isotopic equilibrium with soil water. This contrasts with extra-tropical soils in which the δ¹⁸O_P of resin-P matches the expected equilibrium with soil water. Identical above-equilibrium resin-P δ¹⁸O_P values were also found in field soils that did not receive P additions or extra litter. We suggest that the 3.5‰ above-equilibrium δ¹⁸O_P values reflect a steady state between microbial uptake of phosphate (which enriches the remaining phosphate with the heavier isotopologues) and the release of isotopically equilibrated cell internal phosphate back to the soil. We also found that soil nutrient status affected the microbial turnover rate because in soils that had received chronic P addition, the original δ¹⁸O_P signature of the fertilizer was preserved for at least eight weeks, indicating that the off-equilibrium δ¹⁸O_P values produced during microbial phosphate turnover was not imprinted in these soils. Overall, our results demonstrate that ongoing microbial turnover of phosphate mediates its biological availability in lowland tropical soils.     

Effect of soil air volume change on CH4 consumption in brown forest soil

The relationship between soil air volume and CH₄ consumption in brown forest soils was investigated. The CH₄ consumption was negatively affected when the soil air volume decreased in experiments carried out bothin situ and in the laboratory. A strong dependence of the CH₄ consumption on the soil air volume was found in the laboratory experiment. These results indicate the possibility that the soil air volume is the primary rate-limiting factor for the CH₄ consumption in forest soil. Thus our work suggests that the other factors by which CH₄ consumption might be affected should be examined under the same air volume condition in the soil. To obtain accurate data, we need to be careful not to change the air volume by trampling or disturbing during the measurement of the natural methane fluxin situ.