Long-term fertilization effects on soil organic matter quality and dynamics under different production systems in semiarid Pampean soils

Soil quality can be determined by monitoring, over time, specific chemical, physical or biological properties. Agronomists have long recognized the benefits of maintaining and increasing soil organic carbon (SOC), as well as its labile or particulate (POC) and humic acid (HA) fractions. This research was aimed at the evaluation of continuous wheat (WW) and wheat-grazing (WG), non-fertilized (nf) and annually fertilized with N + P (f), effects on the quantity, quality and dynamics of the organic fractions. The soil (0-0.15 m) was sampled during 1991, 1992, 1993, 1994, 1995 and 1997. Fertilizer application increased labile organic fraction contents compared with the recalcitrant ones. Fertilization increased POC from 0.11 to 0.19% in WW and from 0.15 to 0.20% in WG, while fulvic acid carbon was increased from 0.083 to 0.167% and from 0. 101 to 0.140% in WW and WG, respectively. The humified fraction quantity and quality were modified by fertilization. Quantity differences depended on production system, mainly, animal grazing activity. Changes in HA quality were related to higher labile OC content. The HA extracted from fertilized soils showed higher aliphatic and phenolic -OH groups contents than unfertilized ones, associated to high labile organic fractions contents. The POC dynamics suggested a strong dependence to annual precipitation, mainly water availability during the fallow period. These results confirm the important effect of the long-term fertilization on POC content and HA quality. They could be utilized as soil quality indicators, however, soil physical and climatic conditions must be considered. (c) 2005 Elsevier B.V. All rights reserved.