Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Qiyang.

<p>Simulated and measured soil organic carbon (SOC) stocks (0–20 cm) under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at Qiyang.</p

Soil organic carbon in the (1) active, (2) slow, and (3) passive pool under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Soil organic carbon in the (1) active, (2) slow, and (3) passive pool under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

Average monthly precipitation, maximum (Tmax) and minimum (Tmin) temperatures from 1990 to 2010 at (A) Changping, (B) Yangling, and (C) Qiyang.

<p>Error bars are standard deviations for the mean values from 1990–2010.</p

Land management practices for the long-term experimental sites during the different blocks and periods used in the CENTURY model simulations.

<p>Land management practices for the long-term experimental sites during the different blocks and periods used in the CENTURY model simulations.</p

Correlation between simulated SOC and measured SOC under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Correlation between simulated SOC and measured SOC under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

The pools and flows of carbon in the CENTURY model.

<p>The diagram showed the major factors which control the flows <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095142#pone.0095142-AlisterK1" target="_blank">[29]</a>.</p

Carbon input (kg ha⁻¹yr⁻¹) from manure and straw residue in each period used in the CENTURY model at Changping, Yangling, and Qiyang sites.

<p>Carbon input (kg ha⁻¹yr⁻¹) from manure and straw residue in each period used in the CENTURY model at Changping, Yangling, and Qiyang sites.</p

Soil organic carbon dynamics under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.

<p>Soil organic carbon dynamics under the control, N, NP, NPK, NPKM, hNPKM and NPKS treatments at (A) Changping, (B) Yangling, and (C) Qiyang sites.</p

The relationships between the reduction rate of the contribution of MBN to PMN and the reduction rate of PMN content after POM-removal.

<p>Note: the open symbols represent the soils from the rice-rapeseed rotation and the solid symbols represent the soils from the cotton-rapeseed rotation. The contribution of soil MBN to PMN = MBN / PMN; decrease in the rate of contribution of MBN to PMN = (Contribution _{original soil}—Contribution _{POM-removal soil}) / Contribution _{original soil} × 100; Reduction rate of PMN = (PMN_{original soil}—PMN_{POM-removal soil}) / PMN_{original soil} × 100.</p

Dynamics of Potassium Release and Adsorption on Rice Straw Residue

<div><p>Straw application can not only increase crop yields, improve soil structure and enrich soil fertility, but can also enhance water and nutrient retention. The aim of this study was to ascertain the relationships between straw decomposition and the release-adsorption processes of K⁺. This study increases the understanding of the roles played by agricultural crop residues in the soil environment, informs more effective straw recycling and provides a method for reducing potassium loss. The influence of straw decomposition on the K⁺ release rate in paddy soil under flooded condition was studied using incubation experiments, which indicated the decomposition process of rice straw could be divided into two main stages: (a) a rapid decomposition stage from 0 to 60 d and (b) a slow decomposition stage from 60 to 110 d. However, the characteristics of the straw potassium release were different from those of the overall straw decomposition, as 90% of total K was released by the third day of the study. The batches of the K sorption experiments showed that crop residues could adsorb K⁺ from the ambient environment, which was subject to decomposition periods and extra K⁺ concentration. In addition, a number of materials or binding sites were observed on straw residues using IR analysis, indicating possible coupling sites for K⁺ ions. The aqueous solution experiments indicated that raw straw could absorb water at 3.88 g g⁻¹, and this rate rose to its maximum 15 d after incubation. All of the experiments demonstrated that crop residues could absorb large amount of aqueous solution to preserve K⁺ indirectly during the initial decomposition period. These crop residues could also directly adsorb K⁺ via physical and chemical adsorption in the later period, allowing part of this K⁺ to be absorbed by plants for the next growing season.</p></div