The Influence of Altered Rainfall Regimes on Early Season N Partitioning Among Early Phenology Annual Plants, a Late Phenology Shrub, and Microbes in a Semi-arid Ecosystem

Abstract

In Mediterranean-type ecosystems, nitrogen (N) accumulates in soil during dry summer months and rapidly becomes available during early season rain events. The availability of early season N could depend on the size of rainfall events, soil microbial activity, and phenology of the plant community. However, it is poorly understood how precipitation patterns affect the fate of early season N. Microbes and plants with early phenology may compete strongly for early season N but theory suggests that microbial N storage can meet plant N demands later in the season. Using a 15 N tracer and rainfall manipulation we investigated the fate of early season N. N allocation patterns differed substantially between microbes, early and late phenology plants. As expected early phenology annuals and microbes took up 15 N, within 1 day, whereas a latephenology shrub allocated 15 N to leaves later in the season. We saw no evidence for microbial storage of early season N; the peak of 15 N in shrub leaves did not coincide with detectable levels of 15 N in the microbial biomass or labile soil pool. This suggests that shrubs were able to access early season N, store and allocate it for growth later in the season. Although we saw no evidence of microbial N storage, N retention in soil organic matter (SOM) was high and microbes may play an important role in sequestering N to SOM. Plant N uptake did not respond significantly to 1 year of rainfall manipulation, but microbes were sensitive to dry conditions. 1 year after 15 N addition shrubs had resorbed up to half of the N from leaves whereas N in annuals remained as dead leaf litter. Differences in end-of-season N partitioning between dead and living biomass in the two vegetation types suggest that plant species composition could affect N availability in the following growing season, but it may take several years of altered precipitation patterns to produce rainfall-dependent changes.

Fate of N additions in a multiple resource‐limited Mediterranean oak savanna

“…Nadelhoffer et al (1999) was similar with 45.5% total recovery in an oak forest, despite sampling to 20 cm soil depth. These studies cite various reasoning for their low label recovery such as dry conditions at onset of label application (Hart et al 1993), horizontal mixing with natural abundance material diluting a relatively low-level tracer (Nadelhoffer et al 1999), and sample processing error (Mauritz et al 2014), the first and the last of which are similar to contributing factors in our study. These studies cite various reasoning for their low label recovery such as dry conditions at onset of label application (Hart et al 1993), horizontal mixing with natural abundance material diluting a relatively low-level tracer (Nadelhoffer et al 1999), and sample processing error (Mauritz et al 2014), the first and the last of which are similar to contributing factors in our study.…”

“…Hart et al (1993) had 44% total recovery one year after application to forest soil. Mauritz et al (2014) found just 10-12% recovery in annual plants of a semiarid chaparral system one year after label application. Mauritz et al (2014) found just 10-12% recovery in annual plants of a semiarid chaparral system one year after label application.…”

“…Nadelhoffer et al (1999) was similar with 45.5% total recovery in an oak forest, despite sampling to 20 cm soil depth. Mauritz et al (2014) found just 10-12% recovery in annual plants of a semiarid chaparral system one year after label application. These studies cite various reasoning for their low label recovery such as dry conditions at onset of label application (Hart et al 1993), horizontal mixing with natural abundance material diluting a relatively low-level tracer (Nadelhoffer et al 1999), and sample processing error (Mauritz et al 2014), the first and the last of which are similar to contributing factors in our study.…”

Effects of Drought Manipulation on Soil Nitrogen Cycling: A Meta‐Analysis

“…Drought magnitude and duration should influence whether ecosystems operate as net N sinks or sources. A relatively mild drought would be expected to produce a small pool of bioavailable N (Figure a; Hartmann et al, ), since ecosystem N sinks (e.g., microbial and plant N uptake and soil organic matter) may persist or recover quickly from drought (e.g., de Vries et al, ; Fry, Manning, & Power, ; Homyak et al, ; Mauritz et al, ). As drought severity increases, however, a larger mineral N pool, together with drought‐stressed ecosystem N, sinks may generate responses similar to those following ecosystem disturbance, favoring larger N losses via N gas evasion and leaching upon rewetting (e.g., Evans & Burke, ; Hanan et al, ; Homyak et al, ; Homyak et al, ; Homyak et al, ).…”

Phenological responses to nitrogen and water addition are linked to plant growth patterns in a desert herbaceous community

“…Few studies have addressed the combined effects of N and water addition on reproductive phenology of desert annuals (Cleland et al., ; Gutierrez & Whitford, ; Liu, Monaco et al., ; Mauritz, Cleland, Merkley, & Lipson, ; Sharifi et al., ). Our study shows that the combination of N and water addition only advanced flowering onset time and fruiting time of C. arenarius .…”