Effects of long-term grassland management on the carbon and nitrogen pools of different soil aggregate fractions

Sustainable Grassland-Management Systems and Their Effects on the Physicochemical Properties of Soil

Grassland covers approximately 17.4% of Europe's land area, stores about 20% of the world's soil carbon and has the potential to sequester carbon. With the help of sustainable management systems, grasslands could reduce greenhouse gases and act as a terrestrial sink for atmospheric CO2. In this study, we will investigate the effect of grassland management (cutting, grazing, and a combination of the two) and soil depth (0-10, 10-20, 20-30 cm) on the physical (volumetric water content-VWC, bulk density-BD, porosity-POR, mass consisting of coarse fragments-FC) and chemical properties of soil (organic carbon-SOC, inorganic carbon-SIC, total carbon-STC, total nitrogen-STN, organic matter-SOM, C/N ratio, pH) in Central European lowlands. The management system affected BD, SOC and STN and tended to affect VWC and STC in the first soil depth only. Grazing and the combined system stored greater amounts of STN, SOC and STC and had higher BDs at the surface (0-10 cm) compared to the cutting system. Most soil properties were influenced by soil depth, with C/N ratio and BD increasing and SOC, STC, STN, SOM, VWC and POR decreasing with depth. Our study highlights an opportunity for grassland users to improve soil quality, reduce fossil fuel usage and improve animal welfare through their management systems and argues that systems such as grazing and the combined system should be promoted to mitigate climate change.

Vegetation restoration improved aggregation stability and aggregated-associated carbon preservation in the karst areas of Guizhou Province, southwest China

Background

The change in the soil carbon bank is closely related to the carbon dioxide in the atmosphere, and the vegetation litter input can change the soil organic carbon content. However, due to various factors, such as soil type, climate, and plant species, the effects of vegetation restoration on the soil vary. Currently, research on aggregate-associated carbon has focused on single vegetation and soil surface layers, and the changes in soil aggregate stability and carbon sequestration under different vegetation restoration modes and in deeper soil layers remain unclear. Therefore, this study aimed to explore the differences and relationships between stability and the carbon preservation capacity (CPC) under different vegetation restoration modes and to clarify the main influencing factors of aggregate carbon preservation.

Methods

Grassland (GL), shrubland (SL), woodland (WL), and garden plots (GP) were sampled, and they were compared with farmland (FL) as the control. Soil samples of 0-40 cm were collected. The soil aggregate distribution, aggregate-associated organic carbon concentration, CPC, and stability indicators, including the mean weight diameter (MWD), fractal dimension (D), soil erodibility (K), and geometric mean diameter (GMD), were measured.

Results

The results showed that at 0-40 cm, vegetation restoration significantly increased the >2 mm aggregate proportions, aggregate stability, soil organic carbon (SOC) content, CPC, and soil erosion resistance. The >2 mm fractions of the GL and SL were at a significantly greater proportion at 0-40 cm than that of the other vegetation types but the CPC was only significantly different between 0 and 10 cm when compared with the other vegetation types (P < 0.05). The >2 mm aggregates showed a significant positive correlation with the CPC, MWD, and GMD (P < 0.01), and there was a significant negative correlation with the D and K (P < 0.05). The SOC and CPC of all the vegetation types were mainly distributed in the 0.25-2 mm and <0.25 mm aggregate fractions. The MWD, GMD, SOC, and CPC all gradually decreased with increasing soil depth. Overall, the effects of vegetation recovery on soil carbon sequestration and soil stability were related to vegetation type, aggregate particle size, and soil depth, and the GL and SL restoration patterns may be more suitable in this study area. Therefore, to improve the soil quality and the sequestration of organic carbon and reduce soil erosion, the protection of vegetation should be strengthened and the policy of returning farmland to forest should be prioritized.

Topography-driven differences in soil N transformation constrain N availability in karst ecosystems

Fragile karst ecosystems are characterized by complex topographic landscapes associated with high variations in vegetation restoration. Identifying the characteristics and driving factors of nitrogen (N) availability across the topographic gradient is essential to guide vegetation restoration in karst regions. In this study, we collected soil samples and plant leaves along the topographic gradient (ridge, upper slope, middle slope, and foot slope) of convex slopes in the karst fault basin of southwest China, and determined the indicators reflecting soil N availability, N transformation rates, and their controlling factors. Our results showed that foliar N content and δ15N value, soil inorganic N content and δ15N value, and foliar N:P ratio were substantially lower on the steep hillslopes than on the flat top ridge. Steep slope soils also had a lower enzyme C:N ratio but a higher enzyme N:P ratio than the flat ridge soils. Furthermore, the vector angles calculated by soil extracellular enzyme analysis were below 45o in all studied soils and decreased significantly with increasing slope, indicating that microbial growth was generally limited by N. These results jointly suggest the declines in soil N availability across the topographic gradient, which are further explained by the changes in soil inherent N transformation processes. As the slope became steeper, soil mineralization and autotrophic nitrification (ONH4) rates decreased significantly, while ratio of microbial NH4+ immobilization to ONH4 and NH4+ adsorption rate increased significantly, indicating the decrease in soil inorganic N supply capacity. We further found that deteriorated soil structure, decreased soil organic matter and calcium content, altered microbial abundance, and increased ratios of fungi to bacteria and gram-positive bacteria to gram-negative bacteria were the primary drivers of reduced N transformation rates and N availability across the topographic gradient. Overall, this study highlights the critical role of the topography in controlling soil N availability by regulating N transformation processes in karst regions. The topography should be considered an important factor affecting the functions and services of karst ecosystems.

Does liming grasslands increase biomass productivity without causing detrimental impacts on net greenhouse gas emissions?

Soil acidification has negative impacts on grass biomass production and the potential of grasslands to mitigate greenhouse gas (GHG) emissions. Through a global review of research on liming of grasslands, the objective of this paper was to assess the impacts of liming on soil pH, grass biomass production and total net GHG exchange (nitrous oxide (N₂O), methane (CH₄) and net carbon dioxide (CO₂)). We collected 57 studies carried out at 88 sites and covering different countries and climatic zones. All of the studies examined showed that liming either reduced or had no effects on the emissions of two potent greenhouse gases (N₂O and CH₄). Though liming of grasslands can increase net CO₂ emissions, the impact on total net GHG emission is minimal due to the higher global warming potential, over a 100-year period, of N₂O and CH₄ compared to that of CO₂. Liming grassland delivers many potential advantages, which justify its wider adoption. It significantly ameliorates soil acidity, increases grass productivity, reduces fertiliser requirement and increases species richness. To realise the maximum benefit of liming grassland, we suggest that acidic soils should be moderately limed within the context of specific climates, soils and management.

Forest Conversion and Soil Depth Can Modify the Contributions of Organic and Inorganic Colloids to the Stability of Soil Aggregates

The stability of soil aggregates is critical for maintaining soil structures and is positively correlated with soil resident organic and inorganic colloids. Forest conversion and soil depth affects the formation of soil aggregates; however, the detailed mechanisms involved in their stabilization have not been well investigated. Therefore, to explore the main factors that influence the stability of soil aggregates for different forest types and soil depths, twelve soil samples were collected from four types of forests (native, mixed, Chinese fir, and bamboo forest) and three soil depths (0–10 cm, 10–20 cm, and 20–30 cm) in subtropical forests. The results revealed that the distributions and mean weight diameters (MWDs) of large macroaggregates in the bamboo forest were significantly lower than those in the other forest types at all soil depths (p < 0.05). Organic and inorganic colloids (organically-complexed Fe oxide and fulvic acid) in the soil directly impacted the stability of soil aggregates, while soil properties (e.g., pH and bulk density) indirectly promoted soil aggregate stability through the modification of colloids. In both native and bamboo forests, organic colloids contributed most to the stability of soil aggregates, reaching 80.31% and 61.37%, respectively. The contributions of organic colloids were found to decrease with soil depth, which was primarily due to changes in the organic matter caused by the decomposition of litter. Elucidating and promoting the specific contributions of organic and inorganic colloids on the stability of soil aggregates will be increasingly important for the optimal management of different forest types.

Afforestation influences soil organic carbon and its fractions associated with aggregates in a karst region of Southwest China

Variations in soil organic carbon (SOC) and its fractions within soil aggregates in response to land-use change are important to understand the carbon cycles in terrestrial ecosystem. However, responses of total SOC, SOC fractions, and SOC stability in different soil aggregates to land-use change are less addressed, especially in karst regions with serious land degradation. Therefore, bulk soil samples were collected under four land uses with similar geographical characteristics and previous framing practices including farmland (FL), Bamboo forest (BA), landscape tree planting (LAT), and orange orchards (ORO) in a karst region of Southwest China. Contents of total SOC and three carbon fractions based on their degree of oxidizability (F1, very labile; F2, inert; F3, oxidizable resistant) in bulk soil and different soil aggregates (macro-aggregate, micro-aggregate, and silt+clay fraction) were measured. Afforestation significantly increased contents of total SOC and three carbon fractions in bulk soil and soil aggregates, and the influence was more obvious in macro-aggregate than the other aggregates. Contents of total SOC, F1, F2, and F3 under afforestation land increased by 41.73%, 58.19%, 33.91%, and 40.55%, respectively, in bulk soil, by 55.60%, 79.24%, 121.77%, and 43.30%, respectively, in macro-aggregate, by 52.80%, 33.57%, 20.14%, and 75.02%, respectively, in micro-aggregate, and by 26.21%, 35.60%, 29.26%, and 23.75%, respectively, in silt+clay fraction than those under FL. In bulk soil and soil aggregates, proportions of F1, F2, and F3 in total SOC ranged from 0.11 to 0.18, from 0.13 to 0.22, and from 0.60 to 0.73, respectively, suggesting that the stable carbon was the predominant carbon fraction in the study area. Afforestation decreased the values of stability of SOC in macro-aggregate and silt+clay fraction, while it increased the value in micro-aggregate. Although both BA and ORO had higher SOC content in bulk soil than the LAT, but the SOC stability in bulk soil under BA was significantly lower than that under ORO. In conclude, afforestation form FL improved SOC content and altered SOC stability in bulk soil and soil aggregates, and conversion of FL to ORO might be the best choice to increase SOC sequestration in the four land-use types compared in karst regions of Southwest China.

Factors controlling the spatial variability of soil aggregates and associated organic carbon across a semi-humid watershed

Soil aggregates (SA) play crucial roles in soil organic carbon (SOC) sequestration. Different SA fractions contribute differently to the sequestration of SOC. However, few studies have examined the factors controlling SA fractions and associated SOC contents across a watershed. Soil samples were collected at 0-10 cm (surface layer) and 10-20 cm (subsurface layer) from 88 sites across a semi-humid watershed (1.1 km²) on the Loess Plateau, China. These samples were separated into macroaggregates (MA), microaggregates (MI), and silt + clay fractions (SC) by wet-sieving, and SOC content of each fraction was determined. The objectives were to: 1) investigate the spatial variability of SA fractions and associated SOC contents as well as their main controls across an entire watershed, and 2) explore the linkages between soil aggregation and SOC sequestration. The bulk and aggregate SOC contents of all SA fractions showed moderate variability, with coefficient of variations of 23.3-31.9%. Geostatistical analysis indicated that the spatial patterns of SA fractions and SOC content varied with aggregate size. From combined Spearman's correlation analysis and structural equation modelling, we found that soil texture was an important control on the spatial variability of all SA fractions and associated SOC contents. Vegetation dynamics and management practices associated with land use were also important controls on MA and MI and their associated SOC contents, especially in the surface layer. However, SC and its associated SOC content were more sensitive to eco-hydrological processes related to topography. Among the land uses, grassland had the greatest SOC sequestration potential. The fine roots of herbs can wrap MI in MA and increase SOC content within MA, which is the primary mechanism responsible for SOC sequestration in grasslands. These results indicate that using vegetation with fine root systems for restoration is a good strategy to increase SOC sequestration in this region.

The Edaphic and Vegetational Properties Controlling Soil Aggregate Stability Vary with Plant Communities in an Arid Desert Region of Northwest China

The stability of soil aggregates is the basis for supporting ecosystem functions and related services provided by the soil. In order to explore the mechanism of the influence of soil and vegetation properties on the stability of soil aggregates in desert communities, the particle size distribution and aggregate in different communities were compared, and the contribution of soil physical and chemical properties (soil salinity, soil water content, soil pH, soil organic carbon, soil total phosphorus, soil total nitrogen, etc.) and vegetation properties (species richness, phylogenetic richness, plant height and coverage, etc.) to the stability of soil aggregates was determined by using a structural equation model. The results show the following: Soil water content, organic carbon, and salt in river bank plant communities have significant direct positive effects on the mean weight diameter of soil, with path coefficients of 0.50, 0.11, and 0.24, respectively (p < 0.01). Water also indirectly affects soil stability by affecting plant height, soil salt, and soil organic carbon; species richness and vegetation coverage have significant direct positive effects on the soil stability index, with path coefficients of 0.13 and 0.11, respectively (p < 0.01). In the desert marginal plant community, the plant coverage and species richness have significant positive effects on soil stability, with path coefficients of 0.43 (p < 0.001) and 0.35 (p < 0.001), respectively. Phylogenetic richness has a significant direct negative effect on soil stability (p < 0.05), with an effect value of −0.27. Phylogenetic richness indirectly affects soil stability by adjusting the coverage, with an indirect effect value of 0.23. Moisture, ammonium nitrogen, and nitrate nitrogen have significant direct positive effects on soil stability, with effect values of 0.12, 0.09, and 0.15, respectively. Our research shows that the process of soil stabilization is mainly controlled by soil factors and vegetation characteristics, but its importance varies with different community types.

Long-term (64 years) annual burning lessened soil organic carbon and nitrogen content in a humid subtropical grassland

Burning has commonly been used to increase forage production and nutrients cycling in grasslands. However, its long-term effects on soil organic carbon (SOC) and nitrogen (N) pools within the aggregates and the relation between aggregates-associated SOC and soil CO₂ emissions need further appraisal. This study evaluated the effects of 64 years of annual burning on SOC and N dynamics compared to annual mowing and undisturbed treatments in a grassland experiment established in 1950. Soils were sampled from four depths representing the upper 30 cm layer and fractionated into macroaggregates, microaggregates and silt + clay fractions. The macroaggregates were further fractionated into three occluded fractions. The SOC in the bulk soil and aggregates were correlated to soil CO₂ effluxes measured under field conditions. Compared to the undisturbed treatment, annual burning decreased aggregates stability, SOC and N in the upper 30 cm layer by 8%, 5% and 12%, respectively. Grassland mowing induced greater aggregates stability than burning only in the upper 5 cm. Burning also decreased SOC in the large macroaggregates (e.g., 0-5 cm) compared to mowing and the undisturbed grasslands but proportionally increased the microaggregates and their associated SOC. Soil N associated with aggregates decreased largely following grassland burning, for example, by 8.8-fold in the microaggregates within the large macroaggregates at 20-30 cm compared to the undisturbed grassland. Burning also increased soil CO₂ emissions by 33 and 16% compared to undisturbed and mowing, respectively. The combustion of fresh C and soil organic matter by fire is likely responsible for the low soil aggregation, high SOC and N losses under burned grassland. These results suggested a direct link between grass burning and SOC losses, a key component for escalating climate change severity. Therefore, less frequent burning or a rotation of burning and mowing should be investigated for sustainable grasslands management.

Potential benefits of liming to acid soils on climate change mitigation and food security

Globally, about 50% of all arable soils are classified as acidic. As crop and plant growth are significantly hampered under acidic soil conditions, many farmers, but increasingly as well forest managers, apply lime to raise the soil pH. Besides its direct effect on soil pH, liming also affects soil C and nutrient cycles and associated greenhouse gas (GHG) fluxes. In this meta-analysis, we reviewed 1570 observations reported in 121 field-based studies worldwide, to assess liming effects on soil GHG fluxes and plant productivity. We found that liming significantly increases crop yield by 36.3%. Also, soil organic C (SOC) stocks were found to increase by 4.51% annually, though soil respiration is stimulated too (7.57%). Moreover, liming was found to reduce soil N₂ O emission by 21.3%, yield-scaled N₂ O emission by 21.5%, and CH₄ emission and yield-scaled CH₄ emission from rice paddies by 19.0% and 12.4%, respectively. Assuming that all acid agricultural soils are limed periodically, liming results in a total GHG balance benefit of 633-749 Tg CO₂ -eq year^-1 due to reductions in soil N₂ O emissions (0.60-0.67 Tg N₂ O-N year^-1 ) and paddy soil CH₄ emissions (1.75-2.21 Tg CH₄  year^-1 ) and increases in SOC stocks (65.7-110 Tg C year^-1 ). However, this comes at the cost of an additional CO₂ release (c. 624-656 Tg CO₂  year^-1 ) deriving from lime mining, transport and application, and lime dissolution, so that the overall GHG balance is likely neutral. Nevertheless, liming of acid agricultural soils will increase yields by at least 6.64 × 10⁸  Mg year^-1 , covering the food supply of 876 million people. Overall, our study shows for the first time that a general strategy of liming of acid agricultural soils is likely to result in an increasing sustainability of global agricultural production, indicating the potential benefit of liming acid soils for climate change mitigation and food security.

The limited effects of carbonaceous material amendments on nitrite-oxidizing bacteria in an Alfisol

Carbonaceous materials are soil conditioners that affect nitrogen cycles. However, how carbonaceous materials influence nitrite-oxidizing bacteria (NOB) is yet unclear. In this study, we investigated the NOB community and its potential activities under different treatments (control, biochar, straw, limestone, biochar + limestone, and straw + limestone) in an Alfisol, a type of arable soil depleted in calcium carbonate but enriched in aluminum- and iron-bearing minerals. Treatments with limestone increased soil pH, and straw inputs caused an increment of available potassium (AK). Ammonia (NH₄⁺) was inversely changed under the straw and biochar + limestone amendments. None of the treatments significantly impacted the abundance of Nitrobacter (nxrA) or the potential nitrite oxidation activity (PNO). The abundance of Nitrospira (nxrB) increased in the biochar + limestone-treated samples and was significantly correlated with PNO, pH, and AK. High-throughput sequencing results showed that the α-diversity of NOB did not change in response to the treatments. The dominant Nitrobacter OTUs were affiliated within the Clusters 3, 4, 8, and 9 (a new cluster named in this study), while those of Nitrospira were in the lineage II and Namibian soil cluster 2. The limited compositional variation for Nitrobacter was explained by pH, and that for Nitrospira by pH, TN, and NH₄⁺. Among all available data in this study, the richness of Nitrospira was the most important predictor (73%) for PNO. Therefore, we assumed that the community of nitrite oxidizers (Nitrospira) could be relatively redundant in function, supported by the observation that the carbonaceous inputs did not impact either the potential activity or the α-diversity but did affect the abundance and community composition.

Soil aggregates indirectly influence litter carbon storage and release through soil pH in the highly alkaline soils of north China

Background

Soil aggregate-size classes, structural units of soil, are the important factors regulating soil organic carbon (SOC) turnover. However, the processes of litter C mineralization and storage in different aggregates-size classes are poorly understood, especially in the highly alkaline soils of north China. Here, we ask how four different aggregate sizes influence rates of C release (C_r) and SOC storage (C_s) in response to three types of plant litter added to an un-grazed natural grassland.

Methods

Highly alkaline soil samples were separated into four dry aggregate classes of different sizes (2–4, 1–2, 0.25–1, and <0.25 mm). Three types of dry dead plant litter (leaf, stem, and all standing dead aboveground litter) of Leymus chinensis were added to each of the four aggregate class samples. Litter mass loss rate, C_r, and C_s were measured periodically during the 56-day incubation.

Results

The results showed that the mass loss in 1–2 mm aggregates was significantly greater than that in other size classes of soil aggregates on both day 28 and day 56. Macro-aggregates (1–2 mm) had the highest C_r of all treatments, whereas 0.25–1 mm aggregates had the lowest. In addition, a significant negative relationship was found between C_s/C_r and soil pH. After incubation for 28 and 56 days, the C_s was also highest in the 1–2 mm aggregates, which implied that the macro-aggregates had not only a higher CO₂ release capacity, but also a greater litter C storage capacity than the micro-aggregates in the highly alkaline soils of north China.

Effects of long-term no-tillage systems with different succession cropping strategies on the variation of soil CO2 emission

The optimization of conservationist production systems, whose goal is to increase carbon stocks and reduce greenhouse gas emissions, is considered one of the greatest challenges faced by agriculture nowadays. Therefore, this study aimed to assess the variation of soil CO₂ emission (FCO₂) and its relationship with soil attributes under long-term no-tillage systems with different successions of summer and winter crop sequences. Treatments consisted of combinations of three summer and two winter crop sequences. Summer sequences were maize monocrop (MM), soybean monocrop (SS), and soybean-maize intercrop (SM), while winter crops were crotalaria and maize. FCO₂ showed no difference among summer sequences (p > 0.05). For winter crops, however, the soil under crotalaria crop residues presented higher FCO₂ values (1.03 ± 0.027 μmol m^-2 s^-1) when compared to that under maize crop residues (0.94 ± 0.027 μmol m^-2 s^-1). Soil moisture presented the greatest influence on the temporal variation of FCO₂, being correlated in the summer sequences MM (r = 0.79; p < 0.0001) and SS (r = 0.70; p = 0.002), as well as in the winter crops crotalaria (r = 0.78; p < 0.0001) and maize (r = 0.66; p = 0.005). In the Oxisol under no-tillage for >14 years, the spatial variation of FCO₂ was explained by the soil physical attributes total porosity, macroporosity, microporosity, and soil temperature. The soil under crotalaria crop residues as a winter crop had an improvement in soil physical attributes, leading to a more aerated environment and hence a higher CO₂ production process. However, the winter crops crotalaria (38.65 ± 0.08 Mg ha^-1) and maize (38.14 ± 0.09 Mg ha^-1) also provided a higher carbon stock on this tropical soil. Maize monocrop (41.13 ± 0.11 Mg ha^-1) as a summer crop under no-tillage system also promoted higher carbon stocks on this tropical soil. A strategy to optimize no-tillage systems in terms of FCO₂ reduction and increase in soil carbon stock is related to the adoption of crop cultivation that includes legumes and grasses under intercropping and succession. Therefore, our results suggested that the summer sequences used in this study might contribute to reducing FCO₂ and that both winter crops influenced the increased soil carbon stock.

Distribution and release of phosphorus fractions associated with soil aggregate structure in restored wetlands

Phosphorus, a dominating element responsible for eutrophication, is a potential limiting nutrient in wetland ecosystem. In this study, the release risk of phosphorus was evaluated by investigating the distribution of phosphorus fractions in different grain-sizes of soil aggregates in wetlands with restoration durations of 1, 2, 3, 5, 13 or 19 years. The results showed that the soil aggregate structure tended to be stable when paddy fields were changed into wetland, though aggregate structure first condensed fine-aggregates (<0.25 mm) into coarse-aggregates (>0.25 mm), which was then reversed until the process stabilized after 5 restoration years. With the exception of labile inorganic phosphorus (L-Pi), which continuously decreased within extended abandoned period, total phosphorus (TP), labile organic phosphorus (L-Po), moderately labile organic phosphorus (Ml-Po), iron-aluminum bound phosphorus (Fe·Al-P), calcium-magnesium bound phosphorus (Ca·Mg-P), humic phosphorus (Hu-P) and residual phosphorus (Re-P) concentrations presented a unimodal tendency with a peak at the 2nd or 3rd restoration year, respectively. TP, L-Pi, L-Po and Re-P tended to decrease with decreasing soil aggregate grain-size, and Ml-Po was enriched in small macro-aggregates (0.25-1 mm) and micro-aggregates (0.053-0.25 mm). Macro-aggregates carried Fe·Al-P and Ca·Mg-P. Adsorption isotherm simulation results demonstrated that the retention capacity for phosphorus of a restored wetland first increased and then decreased with extended abandonment period, and macro-aggregates showed a considerable capacity to retain phosphorus. Fe.Al-P and Hu-P had potential release risk with approximate amounts of 197.25-337.25 mg kg^-1 and 131.28-185.72 mg kg^-1, in associated with anaerobic environment and aggregate structure.

Sustainable Conservation Tillage Improves Soil Nutrients and Reduces Nitrogen and Phosphorous Losses in Maize Farmland in Southern China

Monitoring nitrogen (N) and phosphorous (P) losses on farmland is essential for the prevention of agricultural non-point source pollution (NPS). This study was conducted on typical dry farmland in southern China to determine the effect of conservation tillage and conventional tillage (CT) on soil physical and chemical properties, nutrient movement, as well as on N and P losses. Four conservation tillage techniques (i.e., no-tillage direct seeding (NTDS), no-tillage transplanting (NTTS), minimum tillage direct seeding (MTDS), and minimum tillage transplanting (MTTS)), as well as one CT technique, were carried out in a randomized complete block design with three replicates each. The results suggest that MTDS and NTDS improved soil physical and chemical properties by ensuring adequate retention of these properties at the 0–20 cm soil depth. Low levels of N and P losses in runoff and drainage water were recorded under NTTS and NTDS compared to CT. Our results, therefore, suggest that conservation tillage approaches, such as MTDS and NTDS, are the most suitable tillage techniques for improving soil nutrients and reducing agricultural N and P losses while providing an eco-friendly and sustainable agricultural practice.

Kinetic release of arsenic after exogenous inputs into two different types of soil

The mobility of arsenic (As) in soil depends on its sorption/desorption processes on soil particles. Plant uptake locally lowers As concentration in soil pore water, which would trigger resupplies of As from soil solid phase. To better understand the fate of As in soil system after its inputs into soil and its subsequent dynamic processes, diffusive gradients in thin films (DGT) technique along with DGT-induced fluxes in soils (DIFS) model were introduced to study the kinetic information of As in soils, including its response time (T_C) and resupply rate constant (k_-1). To achieve a series of soils with gradient As level, two different types of soils with similar As level (total As in soil JL is 7.4 mg kg^-1, while in soil BJ is 6.5 mg kg^-1) were collected and amended with exogenous As. Then, DGT deployments were carried out following a period of 90-day soil incubation. The simulated T_C values in non-amended soil JL and soil BJ were 0.036 and 0.001 s^-1, respectively. The difference may due to the properties of these two soils, including pH values and contents of adsorption materials, such as Fe and Al compounds. After As inputs into soils, the intrinsic rate of As release from the solid phase to the solution phase in As-amended JL soil was much higher than that in non-amended soil. While for soil BJ, a decreasing trend was observed after As spiking. The redistribution of As may responsible for the different variation trends of As kinetics in these two soils after As spiking. The results indicated that the distribution coefficient of As (K_d) in soil was mainly affected by soil Olsen-P content due to an ubiquitous competition between P and As on soil particles.

Effect of exogenous phosphate on the lability and phytoavailability of arsenic in soils

The effect of exogenous phosphate (P, 200 mg⋅kg-1 soil) on the lability and phytoavailability of arsenic (As) was studied using the diffusive gradients in thin films (DGT) technique. Lettuce were grown on the As-amended soils following the stabilization of soil labile As after 90 days incubation. Phosphate (P) application generally facilitated plant growth except one grown on P-sufficient soil. Soil labile As concentration increased in all the soils after P application due to a competition effect. Plant As concentration increased in red soils collected from Hunan Province, while decreases were observed in the other soils. Even though, an overall trend of decrease was obtained in As phytoavailability along with the increase of DGT-measured soil labile P/As molar ratio. The functional equation between P/As and As phytoavailability provided a critical value of 1.7, which could be used as a guidance for rational P fertilization, thus avoiding overfertilization.