Cristina Mori Alvez

The Uruguayan agricultural sector faces critical environmental sustainability challenges, including erosion and soil organic matter (SOM) losses, nutrient imbalances, and water contamination. Intensification and simplification of production systems exacerbate these issues. This study investigates the impact of agricultural diversification strategies, such as the integration of perennial pastures and cover crops (CCs), on nitrogen (N) and phosphorus (P) dynamics, crop productivity, and the environmental sustainability of agricultural systems. The research outlines three main objectives: (1) evaluating the effect of crop-pasture rotations compared to continuous cropping on N dynamics and wheat yield; (2) assessing the residual effects of two winter legumes (berseem clover and blue lupine) and one grass (black oat) on the subsequent maize crop; and (3) analyzing the ability of legumes from the Lupinus genus to solubilize P in soils with contrasting textures, acidity, and natural fertility. The methodology integrates long-term experiments, on-farm trials, and greenhouse assays, combining conventional and isotopic techniques. Measurements during the study included plant parameters (grain yield, nutrient uptake, above-ground biomass in different plant components) and soil properties. Results show how these strategies enhance agricultural productivity and system sustainability, fostering practices promoting nutrient cycling and resource conservation in Uruguay. Future research directions prioritize understanding the mechanisms behind differences in plant responses to N inputs and P availability. A deeper exploration of processes such as mineralization and immobilization, alongside the influence of microbial communities on nutrient cycling and SOM dynamics, remains essential.

Introduction: Uruguayan agriculture’s transition to no-till farming and intensified practices, replacing crop-pasture (CP) systems with continuous cropping (CC) rotations, has disrupted biological nitrogen fixation (BNF). Despite this, diversified cropping sequences, including C4 species, have maintained the soil organic carbon (SOC) balance under no-till management, with limited overall impacts on productivity and sustainability. The effects of these changes on wheat productivity and nitrogen use efficiency (NUE) need to be further investigated.Methods: This study, conducted within a long-term experiment (LTE) under rainfed conditions, compared wheat productivity in CP and CC rotations. Wheat following CP and CC were analyzed over three seasons. Variables measured included soil nitrogen (N) concentration, wheat grain yield (WGY), grain protein concentration (GPC), and NUE. Four N fertilizer levels were applied to each rotation system to assess their impact.Results: CC rotation consistently outperformed CP in WGY, with 2425 and 1668 kg ha⁻¹ averages, respectively. CP showed slightly higher GPC (10.92%) than CC (10.48%). Nitrate-N levels at tillering positively correlated with WGY and negatively with GPC, but the relationship differed by rotation. Soil NUE indices were higher in CC rotations.Discussion: The study’s findings highlighted the potential of CC rotation, especially when including C4 species in the crop sequence, to achieve higher wheat productivity in the short term due to healthier soil conditions compared to wheat seeded after post-pasture in CP. Additionally, our study highlights that the effect of the previous crop on yield and NUE in wheat was more relevant than the expected residual effect of the pasture phase in CP, primarily due to the quality of residues and the temporary adverse effects of soil compaction caused by livestock trampling.

This study aimed to evaluate the nitrogen (N) residual effects of winter cover crops (CCs) on soil N availability and corn (Zea mays L.) performance over two growing seasons and at two sites in Uruguay. Both conventional and isotopic methods were used to assess the N residual effects of two legume monocultures, a legume–grass mixture, an oat monoculture, and a control without CCs. The experimental design was a randomized block with split plots, where CCs were applied to main plots and N rates (0 and 100 kg ha⁻¹) to subplots. An isotopic trial with ¹⁵N was included to measure fertilizer N use efficiency (NUE). Results varied between sites: at Site 1, legume monocultures enhanced soil N availability and, along with N rate, significantly increased corn yield and N uptake. At Site 2, only the N rate affected these variables. Site 1 had a low crop ¹⁵N recovery, averaging 9.5% due to weeds and heavy rainfall, while Site 2 showed higher recovery, notably when corn succeeded lupine (35%) and mixture CCs (40%). The soil’s top layer and corn grain showed the highest ¹⁵N concentration. The study suggests that specific CC combinations tailored to site conditions may optimize corn yield and NUE.

A substantial amount of phosphorus (P) in the soil is not readily available for plant uptake. Certain species may enhance P availability from poorly soluble P forms. This study focused on improving our comprehension of the effect of two lupine species (L. albus and L. angustifolius) on soil’s P mobilization and its link with soil acidity variations, comparing the response of the lupine species in terms of plant traits (i.e., aboveground biomass and nutrient uptake) with that of oats (Avena strigosa L.) in four contrasting soils (i.e., available P in soil, soil acidity, soil fertility, and texture). The phosphorus solubilization capacity was assessed on variations of P availability (PBray1) at four points in time, comparing soils with lupine to oat-containing soils and their baseline values. Compared to soils containing oats, at harvest, lupine soils had significantly increased PBray1 concentrations; the maximum average increment was around 5.3 mg kg⁻¹, with L. albus in Sites 1 and 2, which presented higher organic matter (OM) contents than the other two sites. Lupine-induced soil acidification did not fully explain that P increase. Oats exhibited the highest increase in shoot dry weight in response to soil’s P availability, while lupine was the least affected. Nevertheless, L. albus showed similar or higher nutrient uptake than oats across all soils. The manganese (Mn) concentration was high in both lupine species’ shoot biomass; however, within each lupine species, across all soil types tested, these legumes had different Mn accumulation levels depending on the soil acidity. Lupinus albus had a higher ability to mobilize non-labile P in the light-textured soil with a high OM content, achieving comparable and higher plant P status than oats and providing N through biological N fixation (BNF), positioning it as a suitable crop for diversifying Uruguay’s agricultural crop rotation systems.

A substantial amount of phosphorus (P) in soil is not readily available for plant uptake. Certain species may enhance P availability from poorly soluble P forms. This research aimed to compare the P solubilization capacity of the soil of two Lupine species, L. albus, and L. angustifolius, with a control crop of oats (Avena strigosa L.). Plants were planted in pots in four soils with varying texture, organic matter content, acidity, and P availability. Phosphorus solubilization capacity was assessed on variations of P availability (PBray1) at four points in time, comparing soils with lupine to oat-containing soils and their baseline values. At harvest, measurements of plant shoot biomass included their shoot dry weight and some nutrient concentrations such as P, nitrogen (N), manganese (Mn), and lupine nitrogen fixation. Compared to soils containing oats, lupine soils significantly increased PBray1 concentrations; increments ranged from 0.9 to 5.4 mg kg-1, depending on species, sampling time, and soil type. Lupine-induced soil acidification did not fully explain this P increase., As a proxy measurement of organic acid exudation, Manganese concentration was high in both lupine species' shoot biomass; however, these legumes had divergent Mn accumulation responses to soil acidity and P availability. Nitrogen fixed quantity varied with Lupine species (105-240 mg pot-1) and soil type (60-240 mg pot-1). Lupinus albus had a higher ability to mobilize non-labile P and provide N through biological N fixation (BNF), positioning it as a suitable crop for diversifying Uruguay's agricultural crop rotation systems.