Long-term straw rather than manure additions improved least limiting water range in a Vertisol

Least limiting water range (LLWR) is a range in soil water content not limiting root growth, which integrates the effect of soil water retention, aeration, and penetration resistance. This study aimed to assess the effect of different long-term fertilization treatments on LLWR in the surface layer of a Vertisol. Soil samples were taken from six long-term fertilization treatments including no fertilization (Control), inorganic fertilization (NPK), inorganic fertilization plus low amount of straw (NPKLS), inorganic fertilization plus high amount of straw (NPKHS), inorganic fertilization plus pig manure (NPKPM) and inorganic fertilization plus cow manure (NPKCM) for measuring LLWR. Results showed that in comparison with the Control, the manure treatments (NPKPM and NPKCM) increased soil organic carbon content (SOC) more pronounced than the straw treatments (NPKLS and NPKHS). However, the water stable aggregates (WSA(0.25)) in the manure treatments was much smaller than the straw treatments. Soil water content at field capacity (theta(FC)) was significantly higher in the manure treatments (NPKPM and NPKCM) than the other treatments. However, soil water content at soil penetration resistance of 3 MPa (theta(PR)) was significantly lower in the straw treatments than in the manure treatments. The LLWR in the manure treatments had sharper decline with bulk density than the other treatments. In comparison with the Control and NPK treatments, the straw treatments significantly increased the LLWR, but manure treatments did not, probably resulting from positive relation between SOC and penetration resistance at a given soil water content and less favorable effect of the manures on soil aggregation than the straw. The S index was significantly increased by the NPKCM treatment relative to the Control, and positively correlated with the SOC. The variation of WSA(0.25) instead of SOC could explain the change of LLWR under different fertilization treatments effectively. These observations imply that the LLWR could assess the effect of organic fertilizations on soil physical quality more comprehensively.

Automated summary

This study assessed the effect of different long-term fertilization treatments on LLWR in a Vertisol. The results showed that manure treatments increased soil organic carbon content, while straw treatments increased water stable aggregates. Additionally, manure treatments increased soil water content at field capacity, while straw treatments reduced water content at soil penetration resistance.