No abstract

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.

determined at planting, and N min is estimated from organic N content multiplied by a mineralization coeffi-Soil N transformations under no-tillage (NT) could affect the utility cient (1-3%). This methodology can be subject to conof the presidedress soil nitrate test (PSNT), particularly where early season growing conditions are cool as in Balcarce, Argentina. The siderable error because a variable fraction of soil PSNT was evaluated for irrigated NT maize (Zea mays L.) with mineral N at planting in humid climates can be lost different N rates (Exp. 1) and for rainfed maize under NT and convenbefore plant uptake (Hergert, 1987). Besides, the magnitional tillage (CT) with different preceding crops and N rates (Exp.tude of N mineralization depends on organic matter 2). In both experiments, N was surface broadcast as (NH 2 ) 2 CO at content, substrate quality and accessibility, soil moisplanting. The reliability of the PSNT was evaluated when NH ؉ 4 -N ture, and temperature (Rice and Havlin, 1994). was determined and when samples were taken up to a 60-cm depth Recent studies using the PSNT have shown a good in the first experiment. In this experiment, the relative yield (RY) correlation between maize grain yield and surface soil was highly correlated (R 2 ϭ 0.61-0.86) with soil NO Ϫ 3 -N concentration NO Ϫ 3 -N content when plants are at V4 to V6 (Magdoff (0-30 cm) at the six-leaf stage (V6). The reliability (R 2 ) did not imet al., 1984; Fox et al., 1989; Binford et al., 1992; Meiprove when NH ؉ 4 -N was determined or when sampling was done up to a 60-cm depth. In the second experiment, there was a good relation-singer et al., 1992b; Sims et al., 1995). Soil NO Ϫ 3 -N conship between the RY and NO Ϫ 3 -N concentration (0-30 cm) at V6 tent at that stage represents the net balance between (R 2 ϭ 0.47), and the R 2 value increased (R 2 ϭ 0.61) when the preceding production (mineralization from soil organic matter, crop was wheat (Triticum aestivum L.). Soil NO Ϫ 3 -N critical concenmanure, and/or fertilizers) and loss (leaching, denitrifitrations ranged between 17 and 27 mg kg Ϫ1 and were associated with cation, and immobilization) because little or no N upa RY of 0.92 or higher in both experiments. The results of this studytake occurs prior to that stage (Meisinger et al., 1992b).show that the PSNT can be used to evaluate preplant N applications Magdoff et al. (1984) have proposed the PSNT as an as a complementary method to N budget in maize under different index of N-mineralization intensity from soil organic management practices.

Soil organic matter (SOM) and especially its labile fractions such as particulate organic matter (POM) are very sensitive to soil use and strongly influence soil ecosystem services. Particulate organic matter has been proposed as a soil quality/health indicator but its determination is tedious and time consuming (i.e. manhours). Anaerobic mineralizable nitrogen (AN) is closely related to the soil organic fraction and is very easily determined. Therefore, we proposed to evaluate AN as a soil quality/health indicator through the assessment of its relationship with SOM, POM, soil aggregate stability (AS), and maize (<em>Zea mays</em> L.) relative yield (RY) under different long term soil uses for cropping at Balcarce, Argentina (37º 45’ 14’’ S, 58º 17’ 52’’ W). Soil samples had been taken at two depths (0-5 and 5-20 cm) in the fall of 1998, 2000, 2003, 2006, 2009, and 2012 from a long term tillage system (TS, conventional (CT) and no-tillage (NT)) and nitrogen fertilization (NF, with and without nitrogen as fertilizer) experiment on a complex of Typic and Petrocalcic Argiudolls. Carbon contents in SOM (SOC), POM (POC) and AN were determined in all soil samples, whereas AS was determined in other soil samples taken in 2006, 2009 and 2012 from the arable layer (0-20 cm). Regardless of TS and NF, SOC, POC and AN decreased with time under cropping at both 5-20 and 0-20 cm. In the uppermost layer (0-5 cm) decreases of all three variables were observed only under CT. Anaerobically mineralized nitrogen variation related to SOC (R² 0.59 - 0.78, <em>P </em>&lt; 0.05) and especially POC (R² = 0.80-0.85, <em>P </em>&lt; 0.05) variations. Likewise, changes in maize RY related better (R² 0.92 and 0.95 (<em>P </em>&lt; 0.05) for CT and NT, respectively) to variation in AN, than to SOC and POC variations. Besides, changes in the aggregate mean weight diameter (DMWD) related acceptably to AN at 0-20 cm (R²= 0.67, <em>P </em>&lt; 0.05) and much better at 0-5 cm (R²= 0.86, <em>P </em>&lt; 0.05). Both coefficients of determination were higher than those obtained relating DMWD to SOC or POC. Given the easiness of its determination, its sensitivity, and that it relates to the variation of different key soil parameters and crop behavior, AN could be proposed as an effective soil quality/health indicator. However, studies should be carried out taking into account a broader range of soil and management situations in order to validate the trends observed in this work.

Monitoring soil health status is imperative to pursue sustainable agriculture. Aggregate stability (AS) is fundamental to define several soil functions and, therefore, physical soil health. The objectives of this work were to (i) evaluate the effect of contrasting cropping systems on AS, soil (SOC) and particulate (POC) organic carbon, and anaerobic nitrogen (AN) both in bulk soil and in macroaggregates (MA), and (ii) assess the relationship between AS and AN both in bulk soil and in MA to facilitate soil physical health monitoring. Aggregate stability, AN, SOC and POC were evaluated at three depths (0–5, 5–20, and 0–20 cm) in a Mollisol of the Southeastern Argentinean Pampas under a long‐term experiment of cropping systems (crop‐pasture rotations under conventional tillage [CT] and no‐tillage [NT]). Bulk‐soil SOC and POC contents and AN showed the effect of cropping systems, especially the effect of crop‐pasture rotation and at 0–5 cm depth. However, NT did not lead to SOC sequestration except at 0–5 cm depth. In turn, pastures in the rotation and NT improved AS. Bulk‐soil AN explained 75, 41, and 71% of AS at 0–5, 5–20, and 0–20 cm depths, respectively, and provides an indication of AS status. Instead, AN in MA did not explain bulk‐soil AS changes as much as bulk‐soil AN, except at 0–5 cm depth. Therefore, it is not worth determining AN in MA. However, routine bulk‐soil AN determination at 0–20 cm depth by producers to diagnose nitrogen soil fertility would also provide an additional valuable indication of AS status.