Inorganic nitrogen, microbial biomass and microbial activity of a sandy Brazilian Cerrado soil under different land uses

The Brazilian Cerrado soils were incorporated into the agricultural production process in the 1970s. The introduction of pastures and/or annual crops utilizing different management systems produced changes in the dynamics of soil organic matter. This study evaluated the microbial attributes of a Typic Quartzipsamment (Arenosols in FAO classification) in native vegetation, pastures, and soybean cultivation under conventional (CT) and no-till (NT) systems. The soil samples (0–5, 5–10 and 10–20 cm layers) were collected in July 2005 and February 2006 from different systems: native Cerrado (CE), CT for 4 years with soybean (CT4S), CT for 4 years with soybean in rotation with millet (CT4S/M), an area that has been under pasture for 22 years (PA22), and an area that remained under pasture for 13 years, followed by NT with soybean in rotation with millet for 5 years (NT5). Soil inorganic N (nitrate and ammonium), microbial C and N and basal respiration were determined. The soil metabolic quotient (qCO2) and the Cmic:Corg ratios were calculated. The predominant form of inorganic N in the native Cerrado (CE) and in the pasture area (PA22) was ammonium, while the conventional system (CT4S/M) and no-till system (NT5) areas presented higher nitrogen availability for crops in the form of nitrate. The microbial C and N concentrations increased in the wet season, and the highest values were found in the Cerrado (CE) and in pasture (PA22) areas, where the permanent soil cover and the lack of soil disturbance by agricultural practices allowed more favorable conditions for microbial development. The CT4S area presented the highest qCO2 index and the lowest Cmic:Ctotal ratio, indicating that the conversion of total carbon into microbial carbon is less efficient in this system. Since sandy soils are more susceptible to degradation, the use of more conservationist management systems promotes more favorable conditions to microbial development and maintenance.