Decomposition and nutrient release characteristics of co-incorporated wheat straw and common vetch in Qinghai-Tibetan Plateau

A mixture of wheat straw and common vetch is widely used for synthetic fertilizers returning to the cropland of the Eastern Qinghai-Tibetan Plateau in China. Their decomposition and nutrient release characteristics are essential to improve soil fertility, while reducing chemical fertilizer application. In this study, a field experiment was performed on the wheat straw (S), common vetch (J) and mixtures in soil by litter bags. Four treatments were set as the wheat straw (S), common vetch (J), the mixture of low ratio of S to J (SH1J), and the mixture of high ratio of S to J (SH2J). The results showed that the decomposition rates were relatively fast at the beginning, and then slowed down gradually. The 0-33 d after incorporation into the soil was the rapid decay stage with the highest decay rate for J (83.8-208.5 mg/d), whereas, the 34-97 d was the slow decay stage with the highest decay rate for S, while the 97-333 d was the steady decay stage with no significant differences for the decay rate among treatments. At the end of decomposition, the decomposition rates of S, J, SH1J, and SH2J were 51.5%, 82.2%, 78.6%, and 75.2%, respectively, indicating that the wheat straw decayed the slowest, while the common vetch decayed the fastest, and the mixtures fell in between. Nitrogen (N) release in the 0-33 d accounted for 70%-83% in the whole decay stages, indicating that N in organic materials released mainly in the first month. At the end of decomposition, the cumulative N release ratios of S, J, SH1J, and SH2J were 21.3%, 81.5%, 79.3%, and 79.0%, respectively. Phosphorus (P) release of common vetch in the 0-33 d and wheat straw in the 0-97 d accounted for 90% and 88% of the total P release, indicating that P in J and S released mainly during 0-33 d and 0-97 d, respectively. At the end of decomposition, the cumulative P release ratios of S, J, SH1J and SH2J were 60.1%, 76.2%, 74.2%, and 82.2%, respectively. As such, the potassium release ratio in the 0-33 d was accounted for more than 95% of the total release ratio. A bi-pool exponential decay model was selected to characterize the decomposition of dry matter, where the N and P had a determination coefficient of above 0.93. The mixture of wheat straw and common vetch increased the labile fraction of N pools, to significantly prolong the average turnover time of labile N. Specifically, the labile fraction of N pool and average turnover time of S were 19.4% and 23 d, while those of CV were 78.8% and 18 d, respectively. The mixture of S and J improved the labile fraction of N pools (14% by SH1J and 25% by SH2J), but prolonged the turnover time (5 d by SH1J and 6 d by SH2J). The mixture of high ratio of S to J also significantly improved the labile fraction of the P pool, 21% higher than the predicted values. Additionally, the K released quickly for both wheat straw and common vetch. The 0-33 d after incorporation into the soil was the rapid decay and nutrient release stage, where the wheat straw decayed and released nutrients the slowest. The mixtures significantly enhanced the labile fraction of N and P pool, thereby prolonging the average turnover time of N, particularly beneficial to nutrient uptake by succession crops. Consequently, the finding can provide promising theoretical and technical support for crop residue incorporation in the cropland of the Eastern Qinghai-Tibetan Plateau in China.