Water controls on nitrogen transformations and stocks in an arid ecosystem

Abstract

Abstract. Following water, nitrogen (N) is the most frequent limiting factor to aboveground net primary production in arid ecosystems. Increased water availability can stimulate both plant nitrogen uptake and microbial nitrogen mineralization, but may also stimulate losses from the ecosystem. Here, we assess the effect of water availability on nitrogen stocks and transformations in an arid ecosystem. We conducted a field experiment with five levels of precipitation input (À80%, À50%, ambient, þ50%, þ80%) and two levels of N fertilization (ambient or 10 gÁm À2 Áyr À1 NH 4 NO 3 ) in a desert grassland of the Chihuahuan Desert. We measured in situ net N mineralization, plant N uptake, foliar N, N leaching under grass-rooting zone, and soil N availability during two years. Our results showed that increased water availability did not affect net N mineralization, but there was higher plant N uptake than with drought. Soil inorganic N pools were 2-4 times lower with increased water availability compared to drought conditions. N leaching below grass-rooting zone was higher in dry than wet conditions because of higher available N. Increased water availability differentially affected N species significantly reducing the NO 3 :NH 4 ratio. The accumulation of inorganic N during drought was the result of a decoupling between microbial and plant activity, and suggests that the cycling of N is more open in dry years than in wet years.

Responses of a desert nematode community to changes in water availability

“…The experiment was conducted in the Jornada Basin, site of the Jornada LTER in New Mexico, USA, and the largest desert in North America, the Chihuahuan Desert, where a long-term experiment to manipulate annual precipitation (PPT) was underway (Reichmann et al 2013). The Jornada desert has three distinct seasons; the hot and wet summer from July to October; the cold and variable winter from November to February; and the hot and dry spring from March to June , Virginia et al 1992.…”

Burrowing herbivores alter soil carbon and nitrogen dynamics in a semi-arid ecosystem, Argentina

“…Nitrogen availability is an important factor controlling productivity of vegetation in semi-arid ecosystems (Lajtha and Whitford, 1989;Reichmann et al, 2013;Yahdjian et al, 2014). Other studies at our site and nearby sites provide information that links belowground activities of vizcachas with vegetation dynamics on burrow systems, and indicate that a portion of the mineralized N in burrow soil is available to deeply rooted shrubs (Villarreal et al, 2008;Hierro et al, 2011).…”

“…Deposition of labile C and N belowground by vizcachas occurs well below the depth of surface litter deposition and decomposition, major processes that contribute to N turnover in soil in semiarid systems (Carrera et al, 2009;Reichmann et al, 2013). Vizcacha feces are a likely source of labile C and N for microbial populations in burrow soil, as indicated by higher rates of decomposition and N subsurface soil from colonial burrows, grazed-only and ungrazed zones incubated at 100% field capactiy and 25 C for 12 weeks, and (b) cumulative potential N min in surface and subsurface soil from colonial burrows, grazed-only and ungrazed zones incubated at 100% field capacity and 25 C for 12 weeks.…”

“…Thus, the relatively high and seasonally stable NO 3 À concentrations in burrows likely originated from accumulation of pulsed mineralization events over time (e.g., Mazzarino et al, 1998;Austin et al, 2004;Yahdjian and Sala, 2010), rather than continuously high in situ rates of net N min . Because potential evaporation exceeds precipitation in this semi-arid region (Paoloni et al, 2003), leaching losses are reduced and NO 3 À accumulation occurs in the absence of plant uptake or microbial immobilization (Mazzarino et al, 1998;Walvoord et al, 2003;Reichmann et al, 2013). Net N min in soil from colonial burrows increased dramatically at soil moisture contents greater than 25% field capacity at both temperatures during laboratory incubations.…”

“…Spatial distribution of soil organic matter is regulated by geomorphic features at large scales and by biotic factors at local scales, while temporal heterogeneity in microbial activity and N min are strongly influenced by pulsed rainfall events (Austin et al, 2004;Yahdjian and Sala, 2010;Reichmann et al, 2013). Plant e soil feedbacks are well understood in shrub-dominated semi-arid ecosystems and can result in patches of C and nutrient-rich soil around woody vegetation embedded in a matrix of soil depleted in C and nutrients (Schlesinger et al, 1996;Mazzarino et al, 1998;Cross and Schlesinger, 1999;Eldridge et al, 2011;Sankey et al, 2012).…”