The effects of compost amendments on rangeland ecosystem services

E. Ashley Shaw, Biology Department, University of Oregon, Eugene, OR, Caitlin T. White and Katharine N. Suding, Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, Holly J. Stover, Environmental Science, Policy and Management, University of California Berkeley, Berkeley, CA, Whendee L. Silver, Environmental Science, Policy, and Management, University of California Berkeley, Berkeley, CA, Lauren M. Hallett, Environmental Studies Program and Biology Department, University of Oregon, Eugene, OR

Background/Question/Methods
Compost amendments are increasingly popular as a soil carbon mitigation strategy on rangelands. As an added benefit, they also increase forage production. Potential mechanisms for this include improved soil water retention (via organic matter) or fertilization from mineralized nutrients. Yet, whether compost improves or hampers ecosystem resilience to rainfall variability is unknown and depends on plant community responses. By retaining soil moisture, compost could buffer plant communities to drought, improving resilience. Alternatively, if fertilization favors competitive, resource-acquisitive over stress-tolerant species, resilience could decline. To test compost’s effects on plant communities and production across rainfall conditions, we use a large-scale field experiment where amendment treatments (compost, inorganic fertilizer, control) are fully crossed with rainfall manipulations (drought via rainout shelters, wet via irrigation, ambient) in California annual grasslands. We matched inorganic fertilizer treatments to compost N mineralization rates to elucidate fertilization from moisture effects. We hypothesized that compost amendments drive positive productivity effects across rainfall conditions by fertilizing under wet/ambient and ameliorating soil moisture in drought treatments. We hypothesized that compost would negatively affect diversity under wet conditions due reduced N and water limitation, leading to dominance of competitive species. We expected no diversity effects under drought due to lower nutrient mineralization.
Results/Conclusions
Our results show that amendment and rainfall treatments interact to alter plant production and community dynamics. Under drought treatments, compost had a positive effect on soil moisture compared to unamended plots. Regardless of rainfall treatment, production was greater in compost compared to control and fertilizer treatments, while the latter two did not differ. There were no differences in production or plant diversity across rainfall treatments. However, diversity was greater in compost compared to fertilizer and control treatments, which may have been due to community reorganization as we found increased turnover in the compost compared to control treatments. By the second year, competitive annual grasses were negatively affected by drought across amendment treatments while more stress-tolerant N-fixers and forbs increased in the compost and fertilized treatments under drought, respectively. We found no effects of amendments on functional groups under the wet rainfall treatment. Overall, we show that compost amendments are promising for increasing soil moisture retention and production, while retaining diversity, especially under drought conditions. As compost amendments become a more common practice across rangelands throughout the US, it is critical to understand how their nutrient and water dynamics feedback to ecosystem resilience.