Critical soil organic carbon for improving available water content of silt loam soils

Abstract The effect of soil carbon on the available water content (AWC) has garnered considerable attention. An increasing number of studies have recognized a beneficial impact of soil organic matter (SOM) on available water content (AWC); however, results are not consistent regarding the magnitude of soil organic carbon (SOC) effect on AWC. In particular, the critical SOC range necessary for enhancing AWC requires clarification. Accordingly, 165 samples were collected from four sites, where two sites received poultry litter (PL) and two sites did not, covering a wide range of carbon contents from low to high (4.5–22.9 g kg −1 ). The bulk density was the same, and the soils were tested for their field capacity (FC) and permanent wilt point (PWP) in the lab. The results revealed that there was no significant correlation between the clay and AWC, and a significant positive relationship between SOC and AWC was exhibited, suggesting that the increase in AWC mostly resulted from the increase in SOC. However, the AWC did not change significantly with increasing SOC when the SOC was below 10 g kg −1 , whereas a significant increase in AWC occurred when the SOC increased from 10 to 14 g kg −1 . The largest increment in AWC occurred when the SOC increased from 14 to 18 g kg −1 , but the increase in AWC with increasing SOC decreased when the SOC was above 18 g kg −1 . Thus, a critical range of SOC for AWC from 14 to 18 g kg −1 could significantly improve the soil water‐holding capacity. Additionally, most soil samples within the critical SOC range were collected from sites that received PL. Thus, this study also evaluated the enhancement of AWC by PL application, which efficiently improved the soil water‐holding capacity.