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Fertilization regimes greatly influence how manganese (Mn) oxides are recycled. However, information on this subject is scarce in rice managed paddies. The objective of this study was to examine the distributions of Mn pools in bulk soil and aggregates under different chemical and organic fertilizer ratios at both surface (0–20 cm) and subsurface (20–40 cm) soil depths. Depth distributions of Mn pools in bulk soil and aggregates were extracted using Mehlich 3 (M3-Mn), dithionite (Mnd), oxalate (Mno), and pyrophosphate (Mnp) solutions under a long-term fertilization experiment in a double rice cropping system. Fertilizer treatments comprised unfertilized control (CK), mineral nitrogen, phosphorus, and potassium (NPK), 30% mineral NPK + 70% organic fertilizer (30F; pig manure + milk vetch), 50% mineral NPK + 50% organic fertilizer (50F), and 70% mineral NPK + 30% organic fertilizer (70F) under a randomly complete block design (RCBD) in paddy conditions. A pattern of 30F > 50F > 70F > NPK > CK was established with positive and significant correlations among total organic carbon (TOC), total nitrogen (TN), and pH. All pools of Mn were in higher concentrations at the subsurface compared to the surface layer, where all aggregate sizes contributed equally to the enrichment of the metal oxides except for Mno in the 0.25–2.0 mm sizes at the surface layer. Distribution of manganese oxides was equal among the different soil aggregates which could cause the microaggregates to transport Mn oxides and pollute both surface and underground water sources.

This research was undertaken (I) to evaluate the status of phosphorus fractions in paddy soils in response to different tillage management practices under different rice-based cropping systems and environments in order to better understand phosphorus behavior in paddy soils and (II) to assess the effects of various tillage practices on crop yield in different soil types and climatic conditions. We selected four tillage treatments, i.e., conventional tillage (T1), conventional tillage with straw return (T2), rotary tillage with straw return (T3), and no-tillage with straw return and autumn plowing with straw return (T4) at Ningxiang and Haerbin, respectively. Soil samples were collected from 20 cm depth, and we investigated soil pH, total P, Olsen P, SOM content, phosphatase activities, phosphorus fractions, and grain yield at the two experimental sites. The results showed that the T4 tillage system led to significantly higher soil organic matter (SOM) content, total P, and Olsen P concentrations at both sites compared with T1. Regardless of the tillage system, the average rice grain yield at Haerbin was 50.3% greater than that at Ningxiang. Phosphomonoesterase (AcP) and phosphodiesterase (DP) enzyme activities were significantly higher, by 39.8% and 62.1% and by 40.3% and 54.6%, under T4 compared to under T1 at Haerbin and Ningxiang, respectively. The organic and inorganic fractions of P were significantly affected by the different tillage systems. Labile and moderately labile P pools were 29.3% and 19.2% higher in Ningxiang and 64.7% and 33.8% higher in Haerbin under T4 compared to T1, and the non-labile P pools were 10.6% and 18.5% lower, respectively. The labile and moderately labile organic P fractions and phosphatase activities showed a significant correlation with SOM and total P concentrations in RDA analysis. Variance partitioning analysis (VPA) showed that different soil properties, climate factors, phosphatase activities, and their interactions were responsible for 7.1%, 6.8%, 1.3%, and 53.7%, respectively, of the variation in grain yield. Different tillage management practices revealed varied effects at both sites for grain yield, P fractions, and phosphatase activities. Tillage management and climatic variations could be the driving factors that influence grain yield in northern and southern parts of China.

Soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) are essential nutrients for plant growth. Thus, soil amendments that increase the amounts of these nutrients can improve crop yield in poor growing soils such as Red soils in subtropical China, which were classified as Plinthosols in the WRB (World Reference Base for Soil Resources). It is unknown whether any of the three nutrients are more influential than the others on crop yield. A 30-year-long field experiment was established in 1986 to investigate the response of grain yield to changes in SOC, TN and TP levels, identified by regression equations, due to various fertilizer treatments applied to corn crops grown in red soil. Corn yields varied significantly under four different fertilizer treatments. For most of the experimental years, the two fertilizer treatments of application of manure combined with inorganic nitrogen, phosphorus, and potassium (NPKM) and a higher rate of NPK (HNPK) fertilizer produced significantly higher yields than the control (no addition of fertilizer, CK) by 441–548 and 380–703%, respectively. Differences between yields increased over time, particularly between NPKM and CK treatments. The long-term application of NPKM increased SOC, TN and TP contents more so than the other treatments and contents were greater than those in the CK by 48.6–57.1, 26.4–37 and 81.6–142%, respectively. There were no significant differences between the treatments with the low and high rates of NPK (LNPK and HNPK). Additional results showed that NPKM raised the C : N ratio and lowered the C : P ratio. The relationships between SOC, TN, TP and grain yield were best represented by the Michaelis–Menten equation rather than the simple linear equation. Moreover, the equations estimated different maximum yields of corn; yields estimated by SOC and TP were higher than the yield estimated by TN. Furthermore, The estimated maximum yield that could be attained is at a C : N ratio of 10.22 in red soil. Therefore, SOC and TP are likely the more important nutrients to consider altering in soil management of red soil.

Soil phosphorus (P) adsorption and desorption occur in an important endogenous cycle linked with soil fertility problems and relevant to the environmental risk assessment of P. In our study, the effect of long-term inorganic and organic fertilization on P adsorption and desorption characteristics in relation to changes in soil properties was evaluated by selecting three long-term experimental sites in southern China. The selected treatments at each site were CK (unfertilized), NPK (synthetic nitrogen, phosphorus and potassium) and NPKM (synthetic NPK plus manure). The adsorption and desorption characteristics of P were evaluated using Langmuir and Freundlich isotherms. The results showed that long-term application of NPK plus manure significantly increased soil organic carbon (SOC), total P and available P at all three sites compared with the NPK and CK treatments. All three treatments fit these equations well. The maximum adsorption capacity (Qm) of P increased with NPKM treatment, and the binding energy of P (K) and the maximum buffering capacity (MBC) showed increasing trends. NPKM showed the highest Qm (2346.13 mg kg-1) at the Jinxian site, followed by Nanchang (221.16 mg kg-1) and Ningxiang (2219.36 mg kg-1). Compared to CK and NPK, the NPKM treatment showed a higher MBC as 66.64, 46.93 and 44.39 L kg-1 at all three sites. The maximum desorption capacity (Dm) of P in soil was highest with the NPKM treatment (157.58, 166.76, 143.13 mg kg-1), showing a better ability to release P in soil. The correlation matrix showed a significant positive correlation of SOC, total and available P with Qm, Dm and MBC. In conclusion, it is suggested that manure addition is crucial to improve P utilization in red paddy soils within the recommended range to avoid the risk of environmental pollution.

Soil enzyme activity and microorganism community can be changed through different long-term fertilization patterns. However, the effect of different fertilization practices on soil microorganisms might differ among crop systems. The objective of the study was to reveal the change of soil enzyme activity and soil microorganism community in different fertilizations both in upland and paddy soils. Therefore, based on long-term fertilization experiments in upland soil started in 1986 and adjacent paddy soil experiment commenced in 1981, with both consisting of 4 treatments: Control (no fertilization), N (only nitrogen fertilizer), NPK (nitrogen, phosphate and potassium fertilizers) and NPKM (nitrogen, phosphate and potassium fertilizers plus organic manure), grain yield, soil fertility, activities of soil urease, catalase, acid phosphatase, microorganism community (the number of bacteria, fungus and actinomycete) were analyzed. The result showed that: the highest grain yield was attained under the application of chemical fertilizers plus manure, as compared with Control, NPKM significantly increased the grain yield by 908.63% in corn and 118.80% in rice (p

It may be good to estimate rice yield based on root bleeding snap weight (BSW) and leaf soil plant analysis development (SPAD) at grain filling stages in southeast China. There were seven treatments: (N-butyl) thiophosphorictriamide (NBPT) applied at rates of 0.00, 0.50, 0.75, 1.00, 1.25, 1.50% with urea @ 135 kg N ha -1 and CK (no NBPT or urea). All treatments received 75 kg P2O5 ha -1 (superphosphate, P2O5 12%) and 150 kg K2O ha -1 (potassium sulfate, K2O 60%), and then all fertilizer and inhibitor were applied once as base fertilizer. At grain filling stage, both of early and late rice, root BSW and leaf SPAD readings were obtained and grain yield determined at harvesting. In addition, the relationship between BSW, SPAD and grain yield was established. The results showed that the grain yield of rice, root BSW and leaf SPAD were increased significantly in application of NBPT with urea. Compared with 0% NBPT, grain yield, BSW and SPAD in 1.00% NBPT were improved by11.9-33.0%, 7.1-74.3% and 4.6-31.7% in early rice; and increased by 8.4-33.0%, 7.9-75.0% and 4.4-19.5% in late rice. However, these decreased when NBPT was less or more than 1.00%. On the other hand, the relationship between BSW, SPAD at grain filling stage and grain yield could be fitted by two-variable linear equation (p

Soil C:N:P stoichiometry plays a vital role in nutrient cycling in ecosystems, but its importance to P transformation in paddy soil remains unclear. We investigated the effect of soil C:N:P stoichiometry on P mobility and uptake under long-term fertilization. Three treatments, CK (no fertilization), NPK (inorganic nitrogen, phosphorus and potassium fertilization) and NPKM (combined inorganic NPK fertilizer and manure application), were selected from two long-term experiments of paddy soil that were initiated in 1991 and 1982 in Chongqing and Suining, respectively. The results showed that in comparison the control treatment, under long-term fertilization, soil pH decreased. In comparison with the NPK and CK treatments, the NPKM treatment significantly increased soil nutrient contents, P uptake and phosphatase activities. In comparison to the CK treatment, the NPK and NPKM treatments significantly decreased soil C:N, C:P and N:P ratios. In comparison to NPK and CK treatments, the NPKM treatment decreased residual-P at both sites. Compared with CK treatment, the NPKM treatments increased labile-P and moderately labile-P by 987% and 144%, respectively, and NPK treatment increased these factors by 823% and 125%, respectively, at the Chongqing site. At the Suining site, with NPKM treatment, increases in labile-P and moderately labile-P were 706% and 73%, respectively, and with NPK treatment, the increases were 529% and 47%, respectively. In contrast, non-labile-P was significantly decreased with NPKM treatment in comparison to that with NPK and CK treatments. Moreover, increases in soil C:N and C:P ratios decreased the labile-P pools and increased non-labile-P pools. A path analysis indicated that soil C:N:P stoichiometry indirectly controlled P uptake by directly affecting P transformation from non-labile to labile-P pools. Moreover, the non-labile-P in soil with high SOM and P content directly affected P uptake, indicating that soil P transformation is mainly driven by soil C and P in paddy soil. In conclusion, understanding mechanism of P mobility influenced by soil C:N:P stoichiometry could be helpful to manage soil P fertility under long-term fertilization in paddy soils of these regions.

Summarization: Soil aggregation and development of soil structure play a crucial role in determining soil functions and ecosystem services in Earth's Critical Zone. Soil organic matter (SOM) is the main constituent that binds mineral particles together into larger-sized aggregates. Many theoretical concepts have been proposed to explain soil aggregation and SOM accrual processes, but quantification of the processes remains lacking. We observed changes in aggregate size mass distribution and aggregate carbon content in 2 discrete years from an 8-year field experiment, which was conducted to determine how to speed up soil development and restoration with different agricultural practices. The 2-year data showed that the proportion of macroaggregates (> 250 μm) increased with the decreasing proportions of microaggregates (53–250 μm) and soil texture units (< 53 μm) and increasing proportion of particulate organic matter occluded in the macroaggregates. This aggregation process was more noticeable in the field treatments with higher organic carbon input. By using these data, we successfully calibrated the recently developed CAST mathematical model, which assumes that primary macroaggregates are first formed around plant-derived organic matter and primary microaggregates are then formed within the macroaggregates as the occluded organic materials are decomposed. The secondary microaggregates and macroaggregates are assumed to form after the breakdown of primary macroaggregates. The calibration processes separated the plant growth season from the frozen winter season. Some calibrated parameters were the same for all of the field treatments, and these parameters included the first-order rate constants of the fragmentation and decomposition of plant litter within the macroaggregates, the decomposition rates of organic matter incorporated and protected in microaggregates, and organic carbon associated with the free soil texture units. Other calibrated parameter values were affected by the field treatments, and the parameters were the first-order decay rate constants of the plant-derived organic matter in nonaggregated soils and in the secondary macroaggregates, the least carbon sources for the formation of the secondary macroaggregates, and the correction factor to adjust carbon with the mass flow of the soil texture units. The differences in the calibrated parameter values suggested that the rates of the primary macroaggregation and the secondary microaggregation were likely controlled by the intrinsic properties of the parent material rather than by the land use and agricultural practices, whereas the rates of the secondary macroaggregation process were likely affected by land use through controlling plant litter quality and quantity and by agricultural practices, such as soil tillage, and to a less extent by fertilizers that control organic input and then carbon and mass content of the soil texture units and microaggregates. The model results were consistent with measured changes over time of the aggregate mass distribution by size class, the total organic matter content, and the SOM bound in aggregates. However, the CAST model did not reproduce particularly well the mass of the soil texture units within different aggregate size classes nor the relative contribution of carbon stocks from different carbon pools within aggregates. The gap between observations and model results points the way forward to develop further the detailed process descriptions in the CAST modeling approach, for example, to consider primary microaggregation directly from basic soil texture units. Nevertheless, this study demonstrates that the CAST model is very powerful to predict the dynamics of soil aggregation and concurrent SOM change in response to changes in land use and soil management. Appearing in: Quantifying and Managing Soil Functions in Earth's Critical Zone Combining Experimentation and Mathematical Modelling

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.