Land use affected nutrient mass with minor impact on stoichiometry ratios in Pampean soils

Abstract

The effect of land use on soil nitrogen (N) and phosphorous (P) stocks and the stoichiometry ratios are not fully understood. We determined the impact of land use on total N and P stocks along with some of their fractions and carbon (C), N and P ratios in soils of the Pampas. The effect of human activities on N and P fluxes in agroecosystems was also assessed. We sampled 386 soils under contrasting land uses down to 1 m depth. Well drained uncultivated soils were used as control treatment, paired with forest, cropped and flooded soils. Significant effects of land use on N and P stocks were detected to 1 m depth. Cropping decreased soil total N and P contents, mineralizable N and extractable P by an average of 14, 21 and 63% respectively. Conversely, forest soils had larger total N stocks (17%), mineralization (10%) and extractable P (37%) than uncropped controls. Flooded lands had the lowest fertility. Nitrogen and P pools under cultivation decreased higher as soils had higher initial levels N and P. In some low fertility soils, cropping led to N and P increases. Stoichiometry ratios were minimally impacted by land use. The ratio of the cumulative P surface balance to the N surface balance for the last 140 years was + 0.01 kg P/kg N in uncropped control soils and − 0.08 kg P/kg N in cropped soils. Despite this difference, the soil N/P ratio was unaffected by land use indicating that processes at the profile level regulated it.