Characteristics of decomposition, nutrient release and structure change of wheat straw in a fluvo-aquic soil under different nitrogen application rates

Objectives Straw decomposition and element transformation are involved in complex biochemical process. Improving soil N content is a key measure to accelerate straw decomposition and nutrient release. Through the research on the decomposition rate, nutrient release and changes in structure and components of straw residues under different application N levels, we can further understand the mechanism of straw decomposition. The outcome through this research would be of great practical significance for high-efficient use of straw resources and sustainable development of farmland.

Methods The experiment site was located in Yuanyang, Henan Province. The soil type was fluvo-aquic soil, and the cropping system was wheat-maize rotation. The wheat straw decomposition was studied using a 187-day straw-bag burying method with three different N fertilizer rates, including 0 (SN0), 180 (SN1) and 240 (SN2) kg/hm². Ultra-high resolution field emission scanning electron microscopy(SEM), solid-state nuclear magnetic resonance (¹³C-NMR) and other methods were used to study dynamic changes of decomposition rate, nutrient release and structure of wheat straw.

Results 1) The decomposition of wheat straw was fast in the early stage and became slow in the later stage. The first two weeks were the rapid decomposition period. The average decomposition rate of straw in this stage was 46%, and that of the whole maize season (100 days) was 71%. High N nutrient level had no significant effect on the decomposition rate of straw in the first two weeks, but the application of N fertilizer accelerated the decomposition of straw after the second week. The decomposition rates of SN1 and SN2 treatments were on average 6 percentage points higher than that of SN0 treatment, but there was no significant difference between SN1 and SN2. The trend of C release rate was basically consistent with the straw mass loss rate. 2) The order of final release rate of straw N, P and K nutrients after 187 days was K (96%‒97%) > N (52%‒86%) > P (29%‒45%), in which K was released completely in the first two weeks, while the release rate of N and P increased negatively in the later period. 3) The decomposition rate of cellulose and hemicellulose was basically consistent with the decomposition rate of straw, also showing a faster decomposition rate in the early stage and a slower one in the late stage. However, lignin decomposed faster in the middle and late stages. The final decomposition rates of cellulose, hemicellulose and lignin after 187 days were 78%‒87%, 86%‒91% and 66%‒73%, respectively. 4) Scanning electron microscopy showed that the structure of wheat straw was gradually destroyed, with the surface becoming rough, faults and cavities being increased, and fiber bundles becoming loose. The straw residues finally formed a network structure. The damage degree of the surface structure of wheat straw under high N rate was more apparent than that under no N treatment. 5) Nuclear magnetic resonance (NMR) results showed that the signal intensities of different organic C functional groups were as follows: O-Alkyl-C (47.02%‒60.13%) > Alkyl-C (11.41%‒17.38%) > di-O-Alkyl-C (10.79%‒13.31%) > Methoxyl-C/N-Alkyl-C (7.53%‒12.02%) > Aryl-C (2.70%‒7.18%) > Carbonyl-C (1.07%‒2.60%) > Phenol-C (0.75%‒2.02%). The relative contents of Alkyl-C, Methoxyl-C/N-Alkyl-C, Carbonyl-C and Phenol-C were significantly increased with straw decompisition, while O-Alkyl-C significantly decreased. 6) Correlation analysis showed that all organic carbon functional groups of straw residues had a significant or extremely significant correlation with the decomposition rate and C release rate. Among all organic C functional groups, only O-Alkyl-C and Methoxyl-C/N-Alkyl-C were significantly related to N release rate. O-Alkyl-C and di-O-Alkyl-C were significantly negatively correlated with the decompositions of cellulose, hemicellulose, and lignin. Moreover, Carbonyl-C and Methoxyl-C/N-Alkyl-C were both highly correlated with lignin degradation rate.

Conclusions The application of N fertilizer could promote the decomposition and C release of wheat straw, and this stimulating effect can only be observed when the straw is returned to soil for two weeks. During decomposition process, the relative content of O-Alkyl-C, which represents a dominant and easily decomposable carbohydrate in the residue of the straw, keeps decreasing with time, suggesting a key role in indicating the process of straw decomposition. Solid-state nuclear magnetic resonance technology shows a good capability to monitor the structural changes of different organic carbon functional groups during straw decomposition, so as to gain a deeper understanding of the mechanism of straw decomposition.