Nitrogen cycling in the soil–plant system along a precipitation gradient in the Kalahari sands

Abstract

Nitrogen (N) cycling was analyzed in the Kalahari region of southern Africa, where a strong precipitation gradient (from 978 to 230 mm mean annual precipitation) is the main variable affecting vegetation. The region is underlain by a homogeneous soil substrate, the Kalahari sands, and provides the opportunity to analyze climate effects on nutrient cycling. Soil and plant N pools, 15N natural abundance (δ15N), and soil NO emissions were measured to indicate patterns of N cycling along a precipitation gradient. The importance of biogenic N2 fixation associated with vascular plants was estimated with foliar δ15N and the basal area of leguminous plants. Soil and plant N was more 15N enriched in arid than in humid areas, and the relation was steeper in samples collected during wet than during dry years. This indicates a strong effect of annual precipitation variability on N cycling. Soil organic carbon and C/N decreased with aridity, and soil N was influenced by plant functional types. Biogenic N2 fixation associated with vascular plants was more important in humid areas. Nitrogen fixation associated with trees and shrubs was almost absent in arid areas, even though Mimosoideae species dominate. Soil NO emissions increased with temperature and moisture and were therefore estimated to be lower in drier areas. The isotopic pattern observed in the Kalahari (15N enrichment with aridity) agrees with the lower soil organic matter, soil C/N, and N2 fixation found in arid areas. However, the estimated NO emissions would cause an opposite pattern in δ15N, suggesting that other processes, such as internal recycling and ammonia volatilization, may also affect isotopic signatures. This study indicates that spatial, and mainly temporal, variability of precipitation play a key role on N cycling and isotopic signatures in the soil–plant system.

Aridity threshold in controlling ecosystem nitrogen cycling in arid and semi-arid grasslands

“…We speculated this to be the main reason as to why the increase of gaseous losses was higher than the increase of net plant N accumulation with increasing AI in areas with AIo0.32. Higher soil N 2 O and other NO X losses from arid land compared with mesic areas have been widely observed 9,34 . Our plant d 15 N and soil nitrification and denitrification gene data provided further evidence to this hypothesis.…”

“…Investigating spatial variability of soil and plant d 15 N across an aridity gradient affords an opportunity to evaluate changes of water-nitrogen interactions 9 . Soil d 15 N has been used as an indicator of the 'openness' of N-cycling in an ecosystem (the ratio of N loss to internally cycled N), because 14 N is preferentially lost from the ecosystem, causing the enrichment of 15 N in soils in ecosystems with higher degree of openness in N-cycling 10 .…”

Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain

“…The extremely high NH 3 concentrations were probably caused by intensified soil emissions after rainfall on 26 July, which enhanced the soil moisture. Precipitation and the resulting soil water dynamics are known to strongly affect urea hydrolysis and subsequent NH 3 emissions (Reynold and Wolf, 1987; Aranibar et al, 2004). The general increase in NH 3 emissions was observed when soils with a high moisture content began to dry because of increased diffusion (Burch and Fox, 1989).…”

Microbial Biobanking Cyanobacteria-rich topsoil facilitates mine rehabilitation

“…It has been well documented that nitrogen fixed by both cyanobacteria and cyanolichens is freely available to higher plants and therefore an important source of plant-available nitrogen (e.g. Aranibar et al, 2004;Evans and Ehleringer, 1994). When it rains, plant-available nitrogen is flushed out of cyanobacterial sheaths into 30 the surrounding soil, where it can be taken up by plants (Dojani et al, 2007).…”