Objectives We studied the nutrient release characteristics of different organic materials in soil to provide a scientific basis for the rational utilization of organic resources.

Methods The straws of potato (Solanum tuberosum L.), rape (Brassica napus L.), wheat (Triticum aestivum L.) and hairy vetch (Vicia villosa Roth.) were loaded into nylon bags and incorporated in 0−20 cm chestnut soil for natural decomposition in Qinghai University. The nylon bags were sampled at 7, 14, 28, 42, 72, 117, and 162 days for determining C, N, and P content, residual straw biomass, and the amount and release rate of C, N, and P were calculated. Further, the relationship between material properties and C, N, and P release rate was quantitatively analyzed.

Results By the end of the decomposition period (162 days), the C residue in the straws of potato, rape, wheat, and hairy vetch were 35.5%, 42.2%, 53.4%, and 14.7%, respectively. During the decomposition process, potato, wheat, and hairy vetch straw had the fastest C mineralization rate at 0−7 days, while rape straw had the fastest C mineralization rate at 7−14 days. The two-pool exponential accumulated temperature model fitted the C residual process well, with R² > 0.90. The annual humification coefficients of potato, rape, wheat, and hairy vetch were 41%, 34%, 58%, and 24%, respectively. During decomposition, wheat straw and hairy vetch continually released N and P, while the N and P in potato and rape straw exhibited eluviation−concentration−release. At the end of the decomposition period (162 days), the total N released from potato, rape, wheat, and hairy vetch straws were 28.0%, 10.2%, 31.8%, and 88.3%, and the total P released was 45.1%, 33.9%, 62.4%, and 71.4%, respectively. The P release rate of potato, rape, and wheat straws was higher than the N release rate, while the N release rate in hairy vetch was higher than the P release rate. Multiple stepwise regression showed that the initial C and N content of the organic materials explained 98.5% of total N release, showing a positive effect, and the C/P ratio explained 82.5% of P release, showing an inhibition effect.

Conclusions The initial C and N content and the C/P ratio of organic materials decide the total release of N and P from them. The humification coefficient of potato, rape, wheat, and vetch straws were 41%, 34%, 58%, and 24%. Hairy vetch has the fastest and direct N and P release in the decomposition process, so it can replace part of the fertilizer. The N and P release rates from potato and rape straws are low, and show a eluviation−concentration−release process during decomposition, they may compete with microorganisms for external N and P nutrients during 14-72 days of decomposition. Therefore, a certain amount of fertilizer should be applied if potato or rape straw is returned to field. Wheat straw has the highest coefficient of humification when returned to field, and most of the N and P is released directly, thus giving the best results in enriching soil.