Spatial heterogeneity and affecting factors of litter organic carbon and total nitrogen over natural spruce-fir mixed forests in northeastern China

Abstract

Litter, as the basic carrier of nutrients, is a link between plants and soil in nutrient cycling and plays a major role in maintaining soil fertility and promoting material cycling and nutrient balance in forest ecosystems. The present study used geostatistics to investigate the spatial heterogeneity of organic carbon (OC) and total nitrogen (TN) in litter and their influencing factors, over four representative 1-ha natural spruce-fir mixed stands in Jilin Province, China. Our results showed that forest litter OC and TN varied in the four stands with an average OC concentration ranging from 378 to 453 g·kg−1 and average TN concentration ranging from 16.4 to 21.8 g·kg−1. The nugget-to-sill values were <25%, revealing that litter OC and TN had a strong spatial autocorrelation. Spatial distributions were estimated using ordinary kriging and showed a distinct strip and patch as the gradient changed. The fractal dimension of litter OC concentration was higher than that of litter TN. This indicated that litter OC had a more complex spatial pattern, whereas litter TN exhibited a better structure and stronger spatial dependence. Stand characteristics (i.e., species number, stem number, biodiversity indices, proportion of conifer species and stems), together with litter properties (i.e., litterfall, litter moisture content and total phosphorous concentration), affected the spatial variation in litter TN concentration, but they had no significant effects on litter OC. Therefore, this study demonstrated the feasibility of using geostatistics to predict litter nutrient concentrations at small scales and provided a theoretical basis for large-scale monitoring of biogeochemical cycles and disturbances in forest ecosystems.