Biological nitrogen fixation by biocrusts and contribution to N flows in a grazed grassland ecosystem in Northern Mexico
Grasslands in Northern Mexico are often seriously degraded because of long-term grazing. These grasslands have low plant cover and extensive vegetation-free interspaces of compacted soils, which are frequently covered by diverse and abundant biocrusts. While their functional role in carbon (C) exchange, infiltration and soil stabilization is fairly well understood, as nitrogen fixing associations little is known with respect to their quantitative contribution to soil N in the surface horizons (0-3.5 cm). During the 2005 dry season, we collected soil biological crust (Acarospora schlecheri, Buellia sp., Disploschistes diacapsis and Placynthiella uliginosa, Acarospora opallens) in the interspaces of an open C4 grassland (Aristida divaricata, Bouteloua scorpioides) and their root associated soil at three depths (0-0.5 cm, 0.5 – 1.5 cm, 1.5-3.5 cm). Natural abundance ratios of the stable isotope of 15N in biocrust and C4 grasses were analyzed. Then, using these as end-members in a mixing model approach, we determined 1) qualitatively to which soil depth soil biological crusts contribute N, and 2) quantitatively the amount of N (total N, and percentage) contributed by different lichen species to each soil layer. We collected root biomass at each corresponding soil depth to examine if biocrust derived soil N stimulates root foraging by neighboring grass roots.
The ∂15N analysis indicated that all lichen species deposited biocrust derived N (apparent N-fixation), in soil beneath the lichen thalli down to 3.5 cm depth. The quantity of biocrust N differed among lichen species (P<0.05). Depending on the lichen species, they contributed from 45 to 57% newly fixed N to the total N pool. A. schleicheri, A. opallens, and Placintiella contributed significantly more N than D. diacapsis and Buellia sp. Hence, these lichen species may contribute to between 0.25 and 0.42 t N ha-1 in the top 3.5 cm soil. Plant root biomass was highest at 0-0.5 cm and declined with increasing soil depth (P<0.05). Root biomass allocation beneath lichen thalli differed among species (P<0.05); it was highest for soils associated with D. diacapsa (P<0.05). Our results suggest that biological soil crusts play a significant role in the N dynamics beyond the biocrust layer, and that these biocrust species may influence soil N fertility at a small spatial scale. For an ecosystem impacted by intensive grazing, roots of grasses within and between these interspatial areas depend on biocrust N-fixation for N provision.