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Long-term soil organic carbon (SOC) and soil organic nitrogen (SON) following cultivation of grassland soils (100/120-year tillage (T) + 20/30-year no tillage (NT)) of the Rolling Pampa were studied calibrating the simple AMG model coupled with the natural13C abundance measurements issued from long-term experiments and validating it on a data set obtained by a farmer survey and by long-term NT experiments. The multisite survey and NT trials permitted coverage of the history of the 140 years with agriculture. The decrease in SOC and SON storage that occurred during the first twenty years by a loss through biological activity was 27% for SOC and 32% for SON. The calibrated model described the SOC storage evolution very well and permitted an accurate simultaneous estimation of their three parameters. The validated model simulated well SOC and SON evolution. Overall, the results analyzed separately for the T and NT period indicated that the active pool has a rapid turnover (MRT ~9 and 13 years, resp.) which represents 50% of SOC in the native prairie soil and 20% of SOC at equilibrium after NT period. NT implementation on soils with the highest soil organic matter reserves will continue to decrease (17%) for three decades later under current annual addition.

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It is expected that the agricultural intensification occurred in recent decades in the Argentine Rolling Pampa significantly alters the SOM reserves. Therefore, it is necessary to identify soil organic carbon (C) and nitrogen (N) fractions to understand the functionality and stabilization of these reserves. Our objectives were to study the NT effect in two crop rotations, corn-double cropped wheat/soybean (MWS) and double cropped wheat/soybean (WS) on: 1) SOM and its particle size and biological fractions contents, 2) C and N stubble biomass and 3) some soil properties in order to explain the SOM differences found. The larger biomass residue remaining on the soil surface under NT promoted higher aggregate stability and lower soil temperature and pH. At 0-5 cm soil depth, NT exhibited higher C and N contents, for both uncomplexed and intimately associated to the mineral components fractions. However, the results indicated variations in the SOM protection according to the rotation: in MWS the high aggregate stability showed better physical protection, while in WS the greater cation exchange capacity and the lower value of N released by anaerobic incubation would indicate the presence of transformed SOM. At 5-20 cm soil depth, only in WS, C microbial biomass was higher with a low metabolic rate, indicating again the presence of highly decomposed SOM. The results obtained in WS under NT would indicate the possibility of achieving slower recycled of the SOM.

Abstract:The soil organic carbon (SOC) of the Argiudolls of the Argentine Rolling Pampa evolves rapidly. Currently, the soils richest in SOC are cultivated with intensified crop sequences (e.g. maize-double cropped wheat/soybean, MWS) under no tillage (NT) and the poorest ones with soybean monoculture (S) under NT. There are great uncertainties about the future projections of SOC reserves and soil fertility associated with changes in land use and management. The aim of this study was to predict soil fertility in 2032, by: a) validating the simple AMG model in long-term experiments of the Rolling Pampa, b) correlating the SOC and active carbon pools (SOCm and Ca, respectively) modeled for 2008 with some soil properties, and c) simulating the evolution of SOC reserves under different agronomic scenarios, using the AMG model, starting from rich and poor SOC soils of the Rolling Pampa. The AMG model was able to provide satisfactory simulation of the SOC reserves (R 2 = 0.87) and showed good quality of fit between Ca and particulate organic carbon (POC) and SOCm and structural stability index (SSI), indicating that the AMG model could project SOC reserves and soil fertility. In rich SOC soils, the maintenance or increase in mean crop yields (MWS NT and MWS with high yields under NT, MWS opt., respectively) caused no changes, whereas the conversion to S under NT reduced the SOC reserves by 12%. Maize residue removal caused 4.5% SOC loss in MWS NT and no changes in MWS opt. In poor SOC soils, the continuity of S under NT and the conversion to MWS NT produced no changes; the passage to continuous or periodic shallow tillage caused 6% SOC loss; and the conversion to Miscanthus x giganteus produced an increase of 9% in SOC.