Yak and Tibetan sheep dung return enhance soil N supply and retention in two alpine grasslands in the Qinghai-Tibetan Plateau

Abstract

Yak and Tibetan sheep grazing is a common phenomenon on natural grasslands in the Qinghai-Tibetan Plateau, and large amounts of excrement are directly deposited onto alpine grasslands. However, little is known about the effects of excrement return on soil N supply and N retention capacity. This study investigated the short-term effects of yak and Tibetan sheep dung on gross N transformation rates determined by N-15 tracing technique of alpine steppe (AS) and meadow (AM) soils at 60 % water holding capacity (WHC) under laboratory conditions. Cumulative gross N mineralization and NH4 (+) immobilization over the 21-day incubation period in AM soil were around 2.8 and 2.0 times as high as that in AS soil, respectively. Cumulative gross nitrification in AM soil was 0.96 times higher, while the value of gross NO3 (-) immobilization rate was 0.65 times lower than in AS soil, resulting in higher NO3 (-) accumulation in AM soil. Dung addition increased soil gross N mineralization and NH4 (+) immobilization rates by 12-35 % and 17-59 %, respectively. Amending yak and sheep dung decreased gross nitrification rates by 3-23 % but increased gross NO3 (-) immobilization rates by 25-190 %, which led to a decreased net NO3 (-) accumulation and NO3 (-) losses risk through leaching. The cumulative CO2 emissions over the 21 days of incubation period were enhanced by 65 and 120 % for AS and AM soil, respectively. The application of dung stimulated cumulative N2O emissions, but the stimulation was only significant in AM soil. In general, yak and sheep dung return has a positive effect on soil N supply and retention owing to the increase in NH4 (+) availability for plant with simultaneously decreasing NO3 (-) accumulation in soils.