Interactive effects of precipitation manipulation and nitrogen addition on soil properties in California grassland and shrubland

Abstract

Soil microbial communities and pools of carbon (C) and nitrogen (N) play an important role in ecosystem responses to precipitation variability and N deposition. In southern California, ecosystem vulnerability to these environmental change drivers may differ for grassland versus shrubland vegetation types. We hypothesized that 1) these vegetation types would differ in their responses to precipitation and N manipulation; 2) reduced precipitation ("drought treatment") would have a negative effect on soil microbial abundance and alter microbial community composition, 3) these changes would be associated with reductions in soil C and N pools, 4) N addition would increase microbial abundance as well as soil C and N pools, and 5) combined drought and N deposition would have offsetting effects on soil properties. We tested these hypotheses at the Loma Ridge Global Change Experiment in southern California. Across vegetation types, we found that microbial biomass based on phospholipid fatty acids declined with drought and N addition. Microbial composition differed more strongly by vegetation type than with environmental change treatments. Added precipitation had little effect on microbial biomass but reduced labile C and N pools; these reductions were mitigated by N addition. Drought reduced labile forms of soil C and N, whereas N addition increased labile soil C pools and all soil N pools. Negative effects of drought and N addition were additive for microbial biomass, which could inhibit soil C cycling if both of these environmental changes occur together. Drought interacted with N addition to significantly increase the most labile N pool under the drought+N treatment, which suggests a build-up of available N under these conditions. These results imply that multiple environmental changes may combine non-additively to affect below-ground microorganisms and soil C and N pools, which may have important consequences for ecosystem services such as productivity, biodiversity, and soil quality in Mediterranean climate regimes of North America.

Impact of understory vegetation on soil carbon and nitrogen dynamic in aerially seeded Pinus massoniana plantations

“…While soil microbial community and enzyme activity could have contributed to changes in soil C and N processes [ 51 – 53 ], which should be considered in further studies examining understory vegetation effects on soil C and N process. In addition, this study was conducted in areas with similar erosion levels before the colonization of both studied understory vegetation types.…”

The effects of heating, rhizosphere, and depth on root litter decomposition are mediated by soil moisture

“…We found that in this Mediterranean grassland ecosystem, soil moisture, which is affected by season, depth, heating, and plants, plays a dominant role in mediating the effect of those factors on root litter decomposition, which after two seasons did not differ by depth or by treatment. Therefore, it will be important to complement this study with others that manipulate soil moisture (e.g., Suttle et al 2007;Khalili et al 2016) to better understand how decomposition of plant litter and soil organic matter will respond and feedback to climate change.…”

Impacts of Global Change on Mediterranean Forests and Their Services

“…This increased risk is a serious concern in arid shrublands and grasslands in southern California with a Mediterranean climate, where the probability of fire increases exponentially above a threshold of N loading [154]. The combined effects of drought and N deposition increases the pools of soil labile N, suggesting a build-up of available N, with unpredictable effects on soil trophic webs, plant-community diversity and C-storage capacity [155]. The potential increase in litterfall production together with the increasing frequency of fire in the Mediterranean area of southern California can favor the spread of shrubs able to sprout over shrubs that depend on seed germination [156].…”