Temporal Effects of Monsoon Rainfall Pulses on Plant Available Nitrogen in a Chihuahuan Desert Grassland

Abstract

Drylands are often characterized by a pulse dynamics framework in which episodic rain events trigger brief pulses of biological activity and resource availability that regulate primary production. In the northern Chihuahuan Desert, growing season precipitation typically comes from monsoon rainstorms that stimulate soil microbial processes like decomposition, releasing inorganic nitrogen needed by plant processes. Compared to microbes, plants require greater amounts of soil moisture, typically from larger monsoon storms predicted to become less frequent and more intense in the future. Yet field‐based studies linking rainfall pulses with soil nutrient dynamics are rare. Consequently, little is known about how changes in rainfall patterns may affect plant available nitrogen in dryland soils, particularly across temporal scales. We measured daily and seasonal responses of soil inorganic nitrogen and related parameters to experimentally applied small frequent and large infrequent rain events throughout a summer growing season in a Chihuahuan Desert grassland. Contrary to long‐standing theories around resource pulse dynamics in drylands, nitrogen availability did not pulse following experimental rain events. Moreover, large infrequent events resulted in significantly less plant available nitrogen despite causing distinct pulses of increased soil moisture availability that persisted for several days. Overall, nitrogen availability increased over the growing season, especially following small frequent rain events that also stimulated some microbial ecoenzymatic activities. Our results suggest that projected changes in climate to fewer, larger rain events could significantly impact primary production in desert grasslands by decreasing plant available nitrogen when soil moisture is least limiting to plant growth.

Long-term fertilization increases soil but not plant or microbial N in a Chihuahuan Desert Grassland

“…Like plants, the unresponsiveness of the microbial community to additional N inputs may be attributed to a possible co-limitation by another resource, especially water but potentially soil phosphorus, potassium or even micronutrients that hinders the biota's capacity to take advantage of additional N (Ramirez et al, 2010;Fay et al, 2015;Radujković et al 2021;Choi et al, 2022). Thus, because N is not being immobilized by plants or microbes, it is likely being lost from the system through leaching during large rain events or volatilization when soils are hot and dry (McCalley and Sparks, 2009;Lovett and Goodale, 2011;Brown et al, 2022). Similar to our results, eight years of simulated increases on N deposition in the Colorado Plateau led to some short-term but no long-lasting effects on an array of soil, plant and microbial metrics, suggesting that serial or simultaneous limitation of multiple other resources may weaken the direct effects of higher N levels on ecosystem processes in drylands (Osborne et al, 2021).…”

“…In general, drylands inherently have high temporal heterogeneity in rainfall, where large, infrequent precipitation events are often thought to regulate ecosystem structure and function (Noy-Meir, 1973), including fluctuations in C and nutrient availability (Collins et al, 2014;Schimel, 2018). In particular, NO3and NH4 + are known to show high intra-annual seasonality in many ecosystems, including drylands where N-availability is often quite low to begin with (Zak et al, 1994) and linked to the timing of rainfall (Cui and Caldwell, 1997;Dijkstra et al, 2012;Brown et al, 2022). Surprisingly, the effect of fertilization did not vary between sampling dates for any of the variables, even when including dates both before and after fertilization.…”

“…Once dry, the sample was combusted in a muffle furnace at 375-450° C for 16 hours. % SOM was calculated as the mass lost by ignition divided by the pre-ignition soil mass following Brown et al (2022).…”

“…There are a number of different ultimate sinks for N added to ecosystems-changing soil nutrient pools, uptake by organisms, or potentially being lost from the system through leaching or volatilization (Peterjohn and Schlesinger, 1990;McCalley and Sparks, 2009;Brown et al, 2022). For example, nutrient cycling models indicate that N inputs of 20 to 35 kg per ha per year in semi-arid ecosystems lead to higher concentrations of NO3leaching beyond the primary rooting zone, a decrease in NH4 + pools in soil, and increasing soil acidity (Fenn et al, 1996.…”

Soil nematode assemblages respond to interacting environmental changes

“…However, the addition of winter rain buffered herbivorous nematodes against declines with nitrogen fertilization. Large rain events, like those added in WENNDEx, reduced soil nitrogen availability in a nearby Chihuahuan Desert grassland dominated only by black grama grass (Brown et al 2022), thus increased rainfall may increase turnover in the microbial loop (Bonkowski 2004), leading to recovery of herbivorous nematode populations.…”