Patricia Satti

Soil nitrogen dynamics in northeastern Patagonia steppe under different precipitation regimes

Small-scale heterogeneity of plant cover and highly variable precipitation events in dry regions can strongly influence N dynamics. We evaluated the differences in N availability (Ni), N mineralization (Nmin), flush of microbial-N (N-MF) and soil moisture (SM) at 0–20 cm depth among four types of patches characteristic of heavily grazed areas in the northeastern Patagonia steppe of Larrea divaricata and Stipa spp. Soil samples were taken monthly during two years of differing annual precipitation (178 mm in 1994 and 325 mm in 1995). Ni and SM were also measured at 20–40 cm depth. Additionally, we estimated the potential N mineralization (pNmin) during two months in both winter and summer in laboratory incubations at 20% soil moisture and 25°C. Sampled patches included: undisturbed patches of shrubs and perennial grasses (GSP), incipient patches of Larrea divaricata and perennial grasses (IGSP), incipient patches of the perennial grass Stipa tenuis (GP), and bare soil (BS). Mineralization rates were much higher during the wet year, and higher in GSP and IGSP than in GP and BS. The prevailing form of Ni was NH4+–N, but pulses of NO3-–N were measured in field incubations when SM was higher than 10%; NO3-–N was also the main form of Ni in pNmin assays. Flush of microbial-N depended mainly on plant cover, following the sequence: GSP>IGSP>GP=BS. It was not correlated with soil moisture, except in the GSP patches, and exhibited lower values during the wet year. Available N (as NH4+–N) was higher in the subsurface than in the surface samples during the wet year. The relative importance of N-MF and Nmin as indicators of spatial and temporal changes in N dynamics, and the role of deep-rooted shrubs in the recovery of soil N fertility, are discussed.

Leaf strategies and soil N across a regional humidity gradient in Patagonia

We analyzed leaf traits related to carbon-fixation, nutrient conservation strategies, and decomposability and their relationships with potential N-mineralization and microbial N in soil in 19 species of 5 dominant life forms growing in 40 sites across a regional humidity gradient in northern Patagonia. We hypothesized that (1) the shifting of species and life forms across the humidity gradient is related to a shifting in traits of green and senesced leaves with some overlapping among life forms, and (2) leaf traits associated with litter decomposition are related to the potential dynamics of soil-N across the humidity gradient. LMA in green leaves and P-resorption efficiency decreased with humidity while concentrations of lignin and total phenolics in green and senesced leaves and P concentration in senesced leaves increased with humidity. Soil C and N concentrations were positively correlated to humidity. Increasing soil N concentration was related to increasing rates of absolute (per unit soil mass) potential net N-mineralization and microbial-N flush. Relative (per unit N mass) potential net N-mineralization and microbial-N flush decreased with soil N and were inversely correlated to lignin concentration and C/N ratio in senesced leaves. We found overlapping in N concentration and C/N ratio in green and senesced leaves, P concentration in green leaves, and N resorption among species and life forms across the humidity gradient. We concluded that (1) leaf traits related to carbon fixation and the decomposition pathway significantly varied with humidity and were not overlapped between plant life forms from dry and humid habitats, (2) the largest overlapping among species and plant life forms across the gradient occurred in those leaf traits related to N conservation in the plant, and (3) life forms from humid habitats produce more recalcitrant litter that induce lower rates of relative potential net N mineralization (per unit N) than those of dry habitats.

Co-composting rice hulls and/or sawdust with poultry manure in NE Argentina.

Rice hulls and sawdust are two common C-rich wastes derived from rice and timber agro-industries in subtropical NE Argentina. An alternative to the current management of these wastes (from bedding to uncontrolled burning) is composting. However, given their C-rich nature and high C/N ratio, adequate composting requires mixing with a N-rich waste, such as poultry manure. The effect of different proportions of poultry manure, rice hulls and/or sawdust on composting efficiency and final compost quality was studied. Five piles were prepared with a 2:1 and 1:1 ratio of sawdust or rice hulls to poultry manure, and 1:1:1 of all three materials (V/V). Different indicators of compost stability and quality were measured. Thermophilic phase was shorter for piles with rice hulls than for piles with sawdust (60 days vs. 105 days). Time required for stability was similar for both C-rich wastes (about 180 days). Characteristics of final composts were: pH 5.8-7.2, electrical conductivity 2.5-3.3 mS/cm, organic C 20-26%, total N 2.2-2.9%, lignin 19-22%, total Ca 18-24 g/kg, and extractable P 6-8 g/kg, the latter representing 60% of total P. Nitrogen conservation was high in all piles, especially in the one containing both C-rich wastes. Piles with sawdust were characterized by high total and available N, while piles with only rice hulls had higher Si, K and pH. Extractable P was higher in 1:1 piles, and organic C in 2:1 piles.

Nutrient availability of composted and noncomposted residues in a Patagonian Xeric Mollisol

Abstract The main objectives of this study were to determine: (1) the agronomic value of composted and noncomposted residues originated in the Andean-Patagonian region (APR), and (2) whether aerobic incubations provide a reliable index of nutrient availability to estimate application rates. A Xeric Mollisol of the transitional zone between the APR and the Patagonian steppe, amended with composted and noncomposted residues, was employed in laboratory incubations without plants and in a 5-month greenhouse trial with ryegrass. Noncomposted residues were biosolids and fish wastes (FW) collected under farming cages. Composts were obtained from fish offal and biosolids mixed with sawdust, woodshavings and yard trimmings. A commercial compost was also included. In laboratory incubations at similar rates of application (10 g kg–1), net N mineralization (Nmin) was about two-fold higher with the noncomposted (17–23%) than with the composted residues (0–12%) and P release was very high with the FW. Ryegrass yields were closely related to the total Kjeldahl N and mineralized N of the organic residues and weakly related to their P contents (total or extractable). Yields were increased even in the case of composts which exhibited very low values of Nmin (and slight N immobilization) during laboratory incubation, suggesting that the presence of plants enhanced Nmin at rates of 10–20 g kg–1. Soil residual extractable P after ryegrass removal was high for fish-derived amendments and the commercial product, suggesting a potential risk of P pollution when these are applied according to N requirements. Aerobic incubations provided a relatively good index with which to assess adequate rates of application.

Agronomic and environmental aspects of utilization of organic residues in soils of the Andean-Patagonian Region

Abstract Patterns of N mineralization and P release were determined in two characteristic soils of the Andean-Patagonian Region (Andisol) and the transitional zone to the steppe (xeric Mollisol) amended with fish residues and biosolids. Residues were applied to the soils at rates of 10–20 g kg-1 and incubated for 112 d under controlled temperature and soil moisture conditions using a non-leaching procedure. Nitrogen mineralized from the total added N depended mainly on the type of residues, while P release depended firstly on the soil type, and secondly on the residue type. Fish residues displayed a potential as slow-release N fertilizers, while biosolids showed a marked initial flush of mineralized N. Due to the high P content, fish residues might exert adverse environmental and agronomic effects on the xeric Mollisol when applied at the rates to meet crop N requirements. In the Andisol with a high P retention capacity, the applied rates would cover both Nand P crop requirements. Biosolids were unable to...