Woody plant reduces soil organic carbon controlled by precipitation.

Woody plant is a significant ecological challenge for grassland ecosystems worldwide. However, changes in soil organic carbon (SOC) fraction contents due to woody plant along a precipitation gradient are poorly understood. This study investigated the characteristics of SOC fractions in shrub-covered grasslands and native grasslands along a precipitation gradient in northern China using a "space-for-time" method, and explored the key factors influencing SOC fractions. The results showed that woody plant decreased total organic carbon (TOC), mineral-associated organic carbon (MAOC) contents and increased dissolved organic carbon (DOC) of grasslands. Mean annual precipitation (MAP) was the main controlling factor influencing SOC fractions, with an explanation proportion of 71.6% (p < 0.01), and the precipitation amount of 112-130 mm was the turning point influencing the direction of variation in SOC fractions due to woody plant of grasslands. With increasing precipitation, the variations in TOC contents of grasslands showed a first increasing and then decreasing trend (R = 0.52), and variations in POC contents showed an opposite trend to those in TOC content (R = 0.59). RDA and PLSPM analyses showed that from native grasslands to shrub-covered grasslands, the main controlling factors for SOC fractions were changed from MAP to a combination of plant properties and MAP, with an explanation of 84.74% and 82.57%, respectively. This study revealed the negative effects of woody plant on SOC of grasslands, and provides new perspectives on the potential dynamics of shrub -precipitation-SOC interactions.