Soil nitrogen availability in olive orchards after mulching legume cover crop residues
Introduction
Nitrogen is the most frequently deficient nutrient in non-legume cropping systems (Havlin et al., 2005). The dynamic of N in the soil-plant system often does not allow its accumulation in the soils in forms readily usable by plants. This requires that N has to be applied every growing season to supplement the lack of naturally available N in soils. The high crop response usually observed after N application has encouraged an excessive use of N-fertilizers in agriculture, reducing N use efficiency and causing diverse environmental problems (Raun and Schepers, 2008). The high price of N and the environmental issues have been pressing for the development of more sustainable farming systems with less reliance on synthetic-N fertilizers.
A reduction in the use of expensive off-farm inputs is of particular importance in marginal agricultural lands, such as the rainfed olive orchards of the Mediterranean basin, having a weak response to external inputs and low returns. Nevertheless, a source of N is always needed, either mineral or organic, without which there will be no proper crop growth and yield. Commercial organic fertilizers are not a solution that could be used widely. Their availability in the market is limited and their prices are high in comparison to their agronomic value (Rodrigues et al., 2006). Farmyard manures and other agricultural and livestock wastes are important fertilizer resources that can fully or partially balance the lack of N in agricultural soils. However, due to the specialization of the agricultural and livestock activities, occurring mainly in the twentieth century, there is a great distance between the sources of organic waste and the soils where they could be recycled. There are nowadays huge environmental problems in regions of intensive livestock production caused by an excessive use of animal waste as a fertilizer (Burton, 2009, Centner, 2011), and, on the other hand, large areas of agriculture where there are not any significant available on-farm sources of organic matter to apply to the soil. In the latter case, a natural source of N must be sought. Legume species, for instance, are able to fix atmospheric N, meeting their own needs and transferring N to a non-legume crop after their N-rich tissues have been mineralized (De Varennes et al., 2007, Russelle, 2008). Legume species can be grown either as main crops in rotation and/or as cover crops or green manures in annual and perennial tree crops.
Organic N sources, such as manures or green manures, are more difficult to manage than the inorganic-N fertilizers, since it is very difficult to predict when their N will become available in the soil. The mineralization of the organic substrates depends on their composition, the C:N ratio in particular (Paul and Clark, 1996, Havlin et al., 2005), and on the environmental conditions affecting microbial activity, such as temperature (Jenkinson and Ayanaba, 1977, Gaiser et al., 1994) and soil moisture (Stanford and Epstein, 1974), making the process unpredictable. It may be, for example, that the N release from the organic residues is not coincident with the periods of active nutrient uptake by the non-legume crops. Furthermore, in particular situations, the environmental damage of using these organic residues can be of similar concern as that associated with the use of synthetic-N fertilizers (Beegle et al., 2008, Sims and Stehouwer, 2008).
In situ incubation methods have been used in the past to monitor N flows in soils under field conditions. They can provide information about the time and rate in which N becomes available in the soil, determining its agronomic and environmental value. The buried polyethylene bag (Eno, 1960, Monaco et al., 2010) and capped PVC or metallic tubes (Raison et al., 1987, Subler et al., 1995, Durán et al., 2012) methods have been the most widely used. Many others, however, have interesting features. An incubation technique using ion exchange resins below intact soil cores for adsorbing inorganic-N leached from the core was used by Di Stefano and Gholz (1986) and Wienhold (2007). Hatch et al. (1990) and Bhogal et al. (1999) incubated soil cores in the field in sealed containers with acetylene to inhibit nitrification and thereby minimize losses of N through denitrification to better measure net rates of mineralization. Rodrigues (2004) incubated soil cores collected by PVC tubes in glass jars buried in the soil to simplify the sampling process and increase the field replications. In general, the techniques using undisturbed soil cores provide reliable quantitative estimates of N transformations in the soil (Raison et al., 1987, Hook and Burke, 1995).
In this study, several legume cover crops were grown in two olive orchards in NE Portugal in an attempt to supply the N needs for the trees. The legume species were cut in spring and left on the ground as a mulch. This would avoid soil tilling and eliminate the consequent damage of the olive tree roots. The dynamic of N in the soil in the year after the establishment of the mulches was monitored by using an in situ incubation technique. The effect of the ground-cover treatments on olive yield and N nutritional status of trees was also determined as an indirect measure of the transfer of N from legumes to olive trees. It is expected that using all these sources of information it will be possible to draw a reliable picture of what happened in the soil when the legume cover crop residues are left on the ground as a mulch.
Section snippets
Site characterization
Two field experiments were carried out in Suçães, Mirandela (41° 29′ N, 7° 15′ W), and Qta do Carrascal, Vila Flor (41° 16′ N, 7° 5′ W), in NE Portugal. The region benefits from a Mediterranean type climate with average annual temperature and precipitation of 14.3 °C and 509 mm, respectively. Weather data recorded in Qta do Carrascal during the experimental period are presented in Fig. 1. The orchard of Suçães was ~20 years old, the olive trees of cv. Cobrançosa and rainfed managed. The soil is a
Results
Dry matter yields of cover crops legumes ranged from 5.6 to 8.2 Mg hm−2 between legume species and 0.7–1.1 Mg hm−2 in natural vegetation plots even in the fertilized one (Table 1). N recovered by the legume cover crops reached 194.6 and 110.2 kg hm−2, respectively in lupine and pasture legume plots of the Suçães experiment, whereas unfertilized natural vegetation recovered only 7.1 kg N hm−2. In Carrascal, lupine, pasture legumes, hairy vetch and natural vegetation recoveries, were respectively, 138.5,
Discussion
From fresh and incubated soil samples a small peak in organic-N was observed in June 2010, shortly after the cutting of the cover crops, and an important peak early in the autumn following the first rains. In the fresh soil samples, the soil inorganic-N levels decreased thereafter throughout the autumn, likely due to N uptake by weeds germinating following the first autumn rains, N uptake by olive trees and nitrate leaching. In incubated soil samples the inorganic-N levels persisted high during
Conclusions
The legume cover crops increased the inorganic-N in the soil and consequently the N nutritional status of the trees and olive yields in comparison to the natural vegetation which was not fertilized. However, the effect on soil available N was slight and short-lived, taking into account the amount of N present in the phytomass when the mulches were established. The reduced efficiency of N from the phytomass in promoting olive yield and tree N nutritional status might be due to: (i) a great peak
Acknowledgements
Supported by FEDER funds through the Operational Program for Competitiveness Factors – COMPETE and National Funds through FCT – Foundation on Science and Technology under the project PTDC/AGR-AAM/098326/2008. The authors thank Rita Diz, Ana Pinto and José Rocha for laboratorial assistance.
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