Effects of small-scale patchiness of alpine grassland on ecosystem carbon and nitrogen accumulation and estimation in northeastern Qinghai-Tibetan Plateau

Small-scale patchiness intensified the spatial heterogeneous distribution of soil and vegetation organic carbon (C) and total nitrogen (N), thereby inducing large uncertainty in estimating ecosystem C and N stocks. However, patchiness at this scale was difficult to identify by satellite remote sensing and measure in situ. Aerial photographing can provide high-precision images and can potentially improve estimating accuracy in ecosystem C and N stocks. In this study, using combined methods of field sampling, aerial photographing and satellite images, we investigated the characteristics of patchiness, quantified ecosystem C and N stocks in different patches at plot scale (~ 1000 m2) and eventually estimated the uncertainty in ecosystem C and N stocks caused by small-scale patchiness at basin scale. Results showed that: 1) mosaic of vegetation and bald patches coexisted for swamp meadow and alpine meadow, whereas vegetation presented as spot for steppe meadow and alpine steppe; 2) average ecosystem respiration, C and N stocks in intact grassland patches were 2.05 and 1.65, 1.66 and 1.65, 2.01 and 1.69 times higher than those of large and medium bare patches (P < 0.05); and 3) neglecting the bare patches caused approximate 30%, 15% and 10% overestimation in estimations of ecosystem C emission, C and N stocks. We concluded that small-scale patchiness would result in ecosystem C loss due to decreasing in C input from vegetation and increasing in C output from soil erosion. Our study suggested that integration of field sampling, aerial photographing and satellite images could improve precision in estimation of ecosystem C emission, C and N stocks at basin scale.