Rice (Oryza sativa L.) systems rotated with perennial pastures have intensified in South America to increase annual grain productivity, but the effects on rice yield and soil quality remain poorly understood. We evaluated rice grain yield, crop and pasture biomass production, and soil organic carbon (SOC) and total nitrogen stocks (0-15-cm depth) in three rice-based rotations over 8 yr in Uruguay. Treatments were: (a) rice-pasture [a 5 yr rotation of rice-ryegrass (Lolium multiflorum Lam.)-rice, then 3.5 yr of a perennial mixture of tall fescue (Festuca arundinacea Schreb.), white clover (Trifolium repens L.), and birdsfoot trefoil (Lotus corniculatus L.)], (b) rice-soybean [a 2-yr rotation of rice-ryegrass-soybean (Glycine max [L.] Merr.)-Egyptian clover (Trifolium alexandrinum L.)], and (c) rice-cover crop (an annual rotation of rice-Egyptian clover). Rice after soybean or pasture achieved the highest yield (9.8 Mg ha -1 ), 9% higher than rice after rice in the rice-pasture and rice-cover crop systems. Estimated belowground biomass under rice-pasture (2.7 Mg ha -1 ) was 12 and 42% greater than under rice-cover crop and rice-soybean rotations, respectively. Rice-pasture showed an increase of 0.6 Mg ha -1 yr -1 of SOC; no changes were observed in the intensified rotations replacing pasture with additional rice or soybean. All systems sustained soil total N. These results provide insights for implementing sustainable rice-based rotations, with rice-pasture being the only system that increased SOC while achieving high rice yields and belowground biomass productivity.
Soybean is the main crop in Uruguayan agricultural systems. Integrated crop management practices are critical to increase productivity while reducing climatic vulnerability in soils with restricted water holding capacity. The objective was to evaluate the impact of contrasting crop management practices and water regimes on crop yield potential. We conducted a three year (2012-2013, 2013-2014 and 2014-2015) field scale study on an Abruptic Argiaquoll at INIA Treinta y Tres, Uruguay. In each season, two experiments were set under contrasting water regimes: rainfed (RF) and supplementary irrigation (SI). Each experiment evaluated 4 Maturity Groups (MG): 5.0, 5.5, 6.1 and 6.8; and 4 Plant Populations (PP): 15, 25, 35 and 45 plants m-2. A RCB split plot design was used and analyzed using mixed models. The greatest productivity was obtained in 2012-2013 (4030 kg ha-1), 22% higher than the average of other seasons. No water regime effect on yield was observed in the first two seasons. In 2012-2013, no yield differences were observed between MG. However, 15 plants m-2 had 4.4% lower yield than other PP (4067 kg ha-1). In 2013-2014 no productivity differences were observed between PP, but MG 5.5 had 11 % higher yield than other MG (3271 kg ha-1). In 2014-2015, SI yield (3760 kg ha-1) was 1020 kg ha-1 higher than RF. SI received 105 mm of supplementary irrigation. In RF, 5.0 MG had a trend of higher yield (16%) compared to other MG (2608 kg ha-1) and 45 plants m-2 decreased yield by 6% compared to other PP (2754 kg ha-1). In SI, the greatest yield was obtained with MG 5.0 (3941 kg ha-1). The aggregate of data suggests that yield potential without water restrictions was 4000 kg ha-1 for most MG, with higher yields in high PP. However, under water limiting conditions in reproductive stages, high populations decreased yield. Water productivity reached 10 kg grain mm-1 applied, reflecting the positive impact of supplementary irrigation.
Carbon net emission is a critical aspect of the environmental footprint in agricultural systems. However, the alternatives to describe soil organic carbon (SOC) changes associated with different agricultural management practices/land uses are limited. Here we provide an overview of carbon (C) stocks of non-forested areas of Uruguay to estimate SOC changes for different soil units affected by accumulated effects of crop and livestock production systems in the last decades. For this, we defined levels based on SOC losses relative to the original (reference) SOC stocks: 25% or less, between 25-50%, and 50% or more. We characterized the reference SOC stocks using three approaches: 1) an equation to derive the potential SOC capacity based on the clay and fine silt soil content, 2) the DayCent model to estimate the SOC stocks based on climate, soil texture and C inputs from the natural grasslands of the area, 3) an estimate of SOC using a proxy derived from remote sensing data (i.e., the Ecosystem Services Supply Index) that accounts for differences in C inputs. Depending on the used reference SOC, the soil units had different distributions of SOC losses within the zones defined by the thresholds. As expected, the magnitude of SOC changes observed for the different soil units was related to the relative frequency of annual crops, however, the high variability observed along the gradient of land uses suggests a wide space for increasing SOC with agricultural management practices. The assessment of the C stock preserved (CSP) belowground and the potential for increasing C sequestration (CAP) are critical components of the C footprint of a given system. In this article we propose a methodological road map to derive indicators of CSP and CAP at the farm level combining both, biogeochemical simulation models and conceptual models based on remotely sensed data. We recognize at least three critical issues that require scientific and political consensus to implement the use of the proposed CSP and CAP criteria: (1) how to define potential or reference C stocks, (2) how to estimate current C stocks over large areas and in heterogeneous agricultural landscapes, and (3) what is a reasonable/acceptable threshold of C stocks reduction.
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