Carbon and nitrogen pools of southern high plains cropland and grassland soils

Soil C and N have long been recognized as important indicators of soil productivity. The current low levels of soil C and N of cropland soils have led to interest in sequestering C with reduced tillage cropping systems and the Conservation Reserve Program (CRP). Our objective was to assess agroecosystem effects on soil C and N pools in the Southern High Plains. The agroecosystems included three cotton (Gossypium hirsutum L.) cropping systems, CRP land, and native rangeland (NR). We sampled 0- to 5-, 5- to 10-, 10- to 15-, and 15-to 30-cm soil depths at 12 farm sites in five counties in West Texas. Total soil C and N, particulate organic matter (POM) C and N, natural abundance of carbon-13 isotope (delta(13)C) of POM and of whole soil, potentially mineralizable C and N, water-extractable carbon (WEC), and extractable ammonium (NW) and nitrate (NO(3)(-) were determined. Total C and N in the 0- to 30-cm soil profile were 34 Mg C ha(-1) and 2.5 Mg N ha(-1) for NR, and 23 Mg C ha(-1) and 1.9 Mg N ha(-1) for cropland systems, respectively. Total soil C and N in CRP land were greater in cropland soils only in the 0- to 5-cm layer, and were 24 Mg C ha(-1) and 2.1 Mg N ha(-1) in 0 to 30 cm. Labile C and N pools were positively correlated with each other and with total soil C and N. Low soil test P may have limited C and N sequestration in CRP land and NR. Improved management practices are needed to sequester C and N in CRP and conservation-tillage cotton systems in the Southern High Plains.