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Soil organic carbon (SOC) is the key to soil fertility and quality. However, SOC has severely declined in the past several decades due in part to inappropriate nitrogen (N) and phosphorus (P) input levels. Thus far, the influences of N and P input levels on the accrual of different SOC fractions and relevant microbial traits at aggregate scales remain uncertain. Here, a 3-year field experiment was used to reveal the effects of N and P input levels on SOC fractions in a Mollisol. Treatments included straw return (SR) and combinations of high nitrogen (N) and high phosphorus (P) inputs (N + P + SR), high N and low P inputs (N + P1/2 + SR), and low N and high P inputs (N1/2 + P + SR). Macroaggregates (>0.25 mm), microaggregates (0.25–0.053 mm), and clay and silt (

The supposition that cropping system might affect fertilization-induced changes in chemical composition of soil organic carbon (SOC) and microbial community composition has mostly derived from field experiments in different sites, which may have been disturbed by different climate conditions and parent materials. Here, two adjacent long-term field experiments were used, which contained similar four fertilization regimes of NPK, 2NPK, NPKOM, and an unamended control (Control), while contrasting annual cropping system of rice–rice and maize–maize, to investigate how fertilization-induced changes in SOC chemistry differ in paddy soil from upland soil, and whether they correlate with different bacterial taxa. The bacterial community composition of the paddy soil was mainly determined by NO3–-N and total N, and that of the upland was mostly explained by SOC. The NPK, 2NPK, and NPKOM treatments from the paddy soil increased CCH3, decreased OCH, and reduced CH/CH2, while those from the upland soil decreased CH/CH2, increased OCH, and raised CH/CH2, respectively, indicating opposite shifts of OCH and alkyl C (i.e., CCH3 and CH/CH2) in each paired fertilizer treatment of the two soils. In the paddy soil, the CCH3 from the NPK treatment showed no association with any bacterial taxa, the OCH from the 2NPK showed a negative association with NO3–-related species, and the CH/CH2 from the NPKOM showed a negative association with Proteobacteria and Bacteroidetes. In the upland soil, the CH/CH2 from the NPK negatively associated with Thermogemmatisporales and Acidobacteriaceae subgroup 1, the OCH from the 2NPK positively associated with Thermogemmatisporales, and the CH/CH2 from the NPKOM positively associated with copiotrophic bacterial taxa. Our results provided direct evidence that cropping system mediated the shift direction of specific functional group under specific fertilization regimes, and implied that completely opposite shifts of OCH and alkyl C in each paired fertilizer treatment of the paddy soil and upland soil can likely be attributed to different fertilization-induced shifts in the microbial community composition resulting from soil N change in the paddy soil and soil C change in the upland soil.

Low-cost, simple, rapid and accurate approaches for measuring soil hydraulic properties are of great importance in the application of hydrologic models of the vadose zone. A recently proposed method of constant-saturation absorption (MCSA) for determining Brooks-Corey (BC) model parameters is promising. However, this method has not been tested using experimental data. In this paper, measured soil-water diffusivity or soil-water retention curves (SWRCs) of 20 soil samples were used to test the reliability of MCSA. The results indicate that MCSA consistently estimated soil-water diffusivities by the Bruce and Klute method, but it substantially overestimated air-entry suctions (h(d)) and thereby soil suction. A new method (MCPA) is proposed herein to overcome the problems associated with MCSA. Using the improved method, estimated SWRCs are consistent with observation. With the estimated soil hydraulic parameters, HYDRUS-1D generated very accurate simulations of the cumulative absorption curves. Compared with direct or other indirect methods, MCPA is a more accurate, low-cost, and simple method, and it is especially useful for rapid determination of SWRC for soil-water movement simulations. However, if unsaturated soil-water diffusivity is of greater concern, our results also indicate that MCSA more accurately predicts this quantity and thus should be preferred. (C) 2015 Elsevier B.V. All rights reserved

Soil organic matter (SOM) and interparticle forces, including the electrostatic repulsive force (ERF), surface-hydration repulsive force (SHRF) and van der Waals attractive force (vDWAF), play crucial roles in aggregate stability. However, few studies investigated their coupled effects on aggregate stability in variably charged soils, in which soil particle interactions are more complex than in permanently charged soils due to the coexistence of positive and negative charges and the variable surface charge characteristics. Therefore, this study aims to: 1) investigate the combined effects of soil solution pH and the overlapping of electric double layers (EDLs) between positively charged Fe/Al (hydro)oxides and negatively charged SOM on the ERF and aggregate stability before (control) and after SOM removal and after straw incubation for 240 days; and 2) evaluate the importance of soil interparticle forces and SOM for aggregate stability in a permanently charged temperate soil (Vertisol) and a variably charged subtropical soil (Ultisol). Soil aggregate stability was determined by measuring the release of small particles (w(

Understanding the balance between soil organic carbon (SOC) accumulation and depletion under different fertilization regimes is important for improving soil quality and crop productivity and for mitigating climate change. A long-term field experiment established in 1989 was used to monitor the influence of organic and inorganic fertilizers on the SOC stock in a soil depth of 0–60 cm under an intensive wheat–maize cropping system in the North China Plain. The study involved seven treatments with four replicates: CM, compost; HCM, half compost nitrogen (N) plus half fertilizer N; NPK, fertilizer N, phosphorus (P), and potassium (K); NP, fertilizer N and P; NK, fertilizer N and K; PK, fertilizer P and K; and CK, control without fertilization. Soil samples were collected and analyzed for SOC content in the 0–20 cm layer each year and in the 20–40 cm and 40–60 cm layers every five years. The SOC stock in the 0–60 cm depth displayed a net decrease over 20 years under treatments without fertilizer P or N, and in contrast, increased by proportions ranging from 3.7% to 31.1% under the addition of compost and fertilizer N and P. The stabilization rate of exogenous organic carbon (C) into SOC was only 1.5% in NPK-treated soil but amounted to 8.7% to 14.1% in compost-amended soils (CM and HCM). The total quantities of sequestered SOC were linearly related ( P − 1 yr − 1 was found to be required to maintain the SOC stock level (zero change due to cropping). However, the organic C sequestration rate in the 0–60 cm depth decreased from 0.41 to 0.29 Mg C ha − 1 yr − 1 for HCM and from 0.90 to 0.29 Mg C ha − 1 yr − 1 for CM from the period of 1989–1994 to the period of 2004–2009, indicating that the SOC stock was getting to saturation after the long-term application of compost. The estimated SOC saturation level in the 0–60 cm depth for CM was 61.31 Mg C ha − 1 , which was 1.52 and 1.14 times the levels for NPK and HCM, respectively. These results show that SOC sequestration in the North China Plain may mainly depend on the application of organic fertilizer. Furthermore, the SOC sequestration potential in the 0–20 cm layer accounted for 40.3% to 44.6% of the total amount in the 0–60 cm depth for NPK, HCM, and CM, indicating that the SOC sequestration potential would be underestimated using topsoil only and that improving the depth distribution may be a practical way to achieve C sequestration.

Soil unsaturated hydraulic properties are an important input for simulations of water and solute movement in the vadose zone. However, in the case of soils containing rock fragments, direct measurement of unsaturated hydraulic properties remains difficult. Recently, an analytical method was proposed (Ma et al., 2009) for determining soil hydraulic properties from horizontal water absorption experiments. Here, we test if this method could be used in the determination of the hydraulic properties of stony soils with a set of experiments. The results show that the method can predict accurately and quickly the hydraulic parameters of stony soils using the cumulative infiltration volume and the rate of wetting front advancement during water absorption into a horizontal soil column. The influence of rock fragment content on some soil hydraulic properties including air entry suction, saturated hydraulic conductivity and the shape coefficients of hydraulic functions was further evaluated. Both air entry suction and shape coefficient show a large range of variation with rock fragment content. Globally, the saturated hydraulic conductivities of stony soils decreased with increasing rock fragment content. However, when the volumetric rock fragment content is about 0.08 cm(3) cm(-3), the stony soil displayed a greater saturated hydraulic conductivity compared to the fine earth at 95% confidence level. The relationship between saturated hydraulic conductivity and rock fragment content is not accurately estimated by different equations relating saturated hydraulic conductivity of stony soils to that of non-stony soils. These findings imply that other factors than the reduction of cross sectional area for water flow influence the hydraulic properties of stony soils. (C) 2010 Elsevier B.V. All rights reserved.

The supposition that phosphate accumulation in soil might result in reduced glyphosate retention capacity and thus increased leaching risk is mostly obtained from many batch experiments, rather than column experiments that may simulate actual field conditions better. However, we hypothesize that: (1) phosphate application might induce system pH decrease, and (2) the overall glyphosate mobility by phosphate application might depend on the relative contribution of two competing processes: increase in glyphosate adsorption from pH decrease versus reduced glyphosate adsorption from competitive adsorption between phosphate and glyphosate for sorption sites available. To test these two hypotheses, laboratory batch and column experiments using a miscible displacement approach were conducted on two Primosols and one Anthrosols,to investigate, respectively, (1) pH-dependent glyphosate adsorption onto the three studied soils, and (2) glyphosate leaching and mobility in soil columns as influenced by phosphate application. Our results showed that glyphosate adsorption consistently decreased with increase in system pH. We found that the extent of phosphate-induced pH decrease was, in decreasing order, Sandic Primosols, Alluvic Primosols, and Stagnic Anthrosols. The effect of phosphate application on glyphosate mobility varied with soil type, with the mobility being reduced in Sandic Primosols, less reduced in Alluvic Primosols, and in Stagnic Anthrosols the mobility seemed to be raised. These differences may be attributed to the relative change of the dominant factor that controlled the glyphosate mobility by phosphate application. In Sandic Primosols the increase in glyphosate adsorption from pH decrease probably dominated over the reduced glyphosate adsorption from the increased repellence between pre-sorbed phosphate and glyphosate; while in Stagnic Anthrosols the later likely became the dominant factor as this soil contained the highest content of soil oxides among the studied soils, which is considered to urge the repellence. We conclude that phosphate application can cause system pH change with various extents in the soil, which subsequently contribute to glyphosate mobility in different degree.