Importance of cyanobacterial crusts as a source of nitrogen of Southern African arid ecosystems, as indicated by acetylene reduction and stable isotopes

J.N. ARANIBAR; I.C. ANDERSON; S. RINGROSE; S.A. MACKO

JOURNAL OF ARID ENVIRONMENTS

Elsevier

Lugar: Holanda; Año: 2003 vol. 54 p. 345 - 358

0140-1963

Cyanobacterial soil crusts may be important in arid and semi-arid ecosystems because of their ability to fix atmospheric nitrogen (N2). These crusts are very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 2). These crusts are very sensitive to trampling by animals, and their destruction can decrease ecosystem N inputs, affecting the productivity of the region. The objective of this study was to quantify the nitrogen-fixing activity in soil crusts during the wet season in southern African ecosystems using in situ acetylene reduction assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 assays. The average acetylene reduction rates for each site ranged from 88 to 535 nmol m2 h1, were highly variable, and were lower than previously reported for other arid areas. All soil samples showed acetylene reduction activity; however, soils with crusts supported higher rates than did ‘‘noncrusty’’ soils under litter, moss, or sand. High values of 15N natural abundance (d15N) indicated that processes other than N fixation were more important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 important in the crusts than N fixation. For example, coupled nitrification/ denitrification and ammonia volatilization or atmospheric deposition of 15Nenriched nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha1 nitrate or ammonium may have caused shifts in d15N within the soil crusts. The estimated annual N fixation rates ranged from 8 to 44 g N ha