Background and aims Reliance on nitrogen (N) fertilizers to maintain crop productivity requires a thorough understanding of the transformation of this nutrient within the soil-plant system. Organic matter input from a mixture of crop residues, such intercrop systems, influence N transformations differently compared to sole crops. We tested the hypothesis that N mineralization and immobilization differ between cereal-legume intercrops and sole crops. Methods A short-term experiment using 15 N isotopic pool dilution was conducted in 2007 and 2012 in maize (Zea mays L.) and soybean (Glycine max L. Merr.) sole crops and 1:2 (1 row maize:2 rows soybean) and 2:3 (two rows maize:3 rows soybean) intercrops. Soil characteristics, gross mineralization and immobilization, and net immobilization to a 10 cm depth were quantified. Results Soil characteristics (pH, bulk density, soil organic carbon (C), total N, and C:N) were not significantly different (P<0.05) among treatments, but differed significantly (P<0.05) between years (2007 vs. 2012). Soil NH 4 + -N was significantly lower (P<0.05) in the maize sole crop. Gross N mineralization, immobilization and net immobilization, were significantly different (P<0.05) among treatments and between years. Relative NH 4 + -N immobilization was significantly different (P<0.05) among treatments and between years, showing the lowest values in the intercrops. The amount of NH 4 + -N mineralized per day was significantly greater (P<0.05) in the 2:3 intercrop and was significantly different (P<0.05) between years in the 2:3 intercrop. Residence time of NH 4 +-N was significantly longer (P<0.05) in the soybean sole crop and 1:2 intercrop followed by the 2:3 intercrop and the maize sole crop. Conclusions Intercropping contributed to the long-term immobilization of N and therefore was a more sustainable land-management practice than sole cropping. The adoption of cereal-legume intercrops will curb our currently growing reliance on N fertilizers.

Total and particulate organic matter content in different aggregate sizes can be used as an indicator of land use effect. We hypothesized that tillage reduction increases total (SOC) and particulate soil organic C (POC) contents and the stability of larger aggregates of high-SOC-content Mollisols. Three management systems (continuous pasture (Pp), and continuous cropping under conventional tillage (CT) and notillage (NT)) were evaluated. Oven dried soil samples (0-5 and 5-20 cm), were wet sieved through 2000, 250 and 50 µm sieves after immersion (IW) and capillary (CW) wetting. Particulate organic matter (>50 µm) was separated in both the whole soil and in each aggregate fraction, SOC and mineral associated organic C contents were determined, and POC was calculated by subtraction. Reduced soil disturbance (Pp and NT) yielded more SOC and POC in the whole soil and macroaggregates with higher stability (Pp>NT>CT). Under Pp, macroaggregates showed the highest SOC and POC. Under NT, macroaggregates showed higher SOC and POC and aggregate stability than CT though lower than Pp. Less tillage-induced disruption (Pp and NT) led to higher SOC and POC contents in more stable macroaggregates but continuous no-tillage appeared not to be enough to reverse the effects of long lasting conventional management practices on these Mollisols.

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<p>El cambio dinámico en el suelo debido a diferentes usos puede monitorearse mediante el seguimiento de variables sensibles. Para el contenido de fósforo (P), no existe un consenso acerca de la variable de medición más adecuada. En este trabajo se evaluó la utilidad de técnicas que miden disponibilidad, pérdida y capacidad de retención de P (P-Bray1, P-CaCl2, P75, ISP y GSP) para diferenciar cambios en el contenido de P del suelo, relacionados al uso de la tierra, así como la relación entre dichas técnicas. Se analizaron dos profundidades de suelo en dos sitios del sudeste de la provincia de Buenos Aires con diferentes secuencias: agrícolas (A), pastizales nativos (PN), sistemas forestales (FO) y pastizales secundarios (PS). Todas las técnicas pudieron diferenciar entre sistemas de manejo; con diferencias variables según la profundidad del suelo. Se detectaron correlaciones estrechas entre las técnicas, resultando dependientes del uso del suelo, P-Bray1 y P-CaCl2 (R2 = 0.89), P75 e ISP (R2 = 0,79) y GSPP75 o GSPISP con P-CaCl2 (R2 = 0.82 y 0.83 respectivamente). Los sistemas A incrementaron la disponibilidad de P (P-Bray1), disminuyeron la capacidad de retención (P75 e ISP) y generaron mayor riesgo de pérdidas de P (P-CaCl2). El FO y la PS presentaron mejor disponibilidad de P respecto de PN, con mayor capacidad de retención y con menores riesgos de pérdidas por escurrimiento. En base a los resultados, podría estimarse una forma de P en función de la determinación analítica de otra, pero de forma diferencial según el uso del suelo.</p>