Mulch and Compost for Restoration of Soil Health in a Semi-Arid Rangeland

Abstract

Between one quarter and one half of dryland (P:PET<0.65) soils are degraded. Soil organic amendments (amendments) can improve soil health, increase plant productivity, and increase soil organic carbon (SOC) content. This has led to a recent surge in research into the use of amendments. However, more research is needed to identify locally appropriate amendments and applications for dryland restoration. My thesis work addresses this need by surface applying (mulching) two locally abundant amendments in a degraded section of a semi-arid rangeland. The amendments used were mesquite branches and a compost of woodchips and manure. The first section of this thesis assesses the effects of amendments on soil health and plant establishment. The second section focuses on the ability of amendments to increase SOC in stable soil fractions that can protect SOC for decades to centuries. We found that mesquite mulch alone increased soil moisture, plant available nitrogen, cover and abundance of plants. However, mulch alone did not increase total soil carbon or carbon content of any measured soil fraction. When combined with compost, total SOC, the carbon in labile pools, and the SOC in stable aggregate and mineral protected pools increased. However, we found that the carbon in protected pools did not resist high levels of dispersion. Furthermore, although compost increased mean soil moisture, it could not do so during dry times of the year in response to small rain events and it. Compost also reduced the cover and abundance of plants at 6 cm applications but did increase the growth and cover of seeded grasses at 3 cm applications. Overall, the use of mesquite branch mulch was found to be highly promising as a method to improve key aspects of soil health and the short-term establishment of plants. Compost on the other hand is not recommended at either 3 cm or 6 cm rates of application, but may be promising at lower application rates. Although longer-term observations are needed to confirm, neither amendment seemed likely to result in persistent soil carbon gains unless careful management actions are taken to prevent soil disturbance and prevent loss of carbon from the weakly bound aggregates and mineral associations we found.