Abstract
In Galicia (NW Spain), kiwifruit (Actinidia chinensis var. deliciosa) is intensively produced using conventional practices (CONV) that ensure high yields, despite the fact that the excessive use of agrochemical inputs leads to soil organic matter depletion, increased water pollution and biodiversity losses. Although more sustainable practices have been implemented in the area (i.e. integrated, INT and organic farming, ORG), it remains unclear how these practices will affect C dynamics mediated by soil biota. Therefore, in this study, we investigated the effects of agricultural management practices and earthworm additions (macrofauna) on soil C losses (CO2 emissions and dissolved organic carbon (DOC)) in relation to the amount of bioavailable soil carbon (salt extractable organic carbon (SEOC) and microbial carbon indicators (microbial biomass or Cmic and the Cmic/Corg ratio)). The experimental design consisted of a 105-days laboratory incubation of intact soil cores collected in the field (40 from each agroecosystem) and earthworm additions over ambient (2.43 ± 0.05 g/earthworm) to half of the experimental units (+EW), with the remaining half acting as controls. Our results showed that earthworm additions led to significant increases in their abundance in all three management treatments, but with the ORG soils sustaining the greatest population sizes. However, no significant effect on soil C transformations were observed in response to these earthworm increases, and instead, legacy agricultural practices overrode macrofauna control on C turnover. Consequently, more C was lost from the CONV treatments than from the ORG ones (on average, 60% more CO2 and 53% more DOC) as a result of CONV practices promoting microbial-mediated processes and hence, amplifying C mineralization versus C stabilization. Furthermore, C release from the INT soils was intermediate between the other two treatments, which suggests that more sustainable farming practices could help in achieving climate change mitigation. These findings provide clear evidence of how local adaptation (at farm level) towards a more environmentally friendly land management could represent a promising strategy to increase soil C sequestration. Future agricultural approaches would need to incorporate the potential benefits from other agroecosystem services beyond those derived from productivity and market values.
Highlights
In this study, we investigated the combined effects of agricultural management practices and earthworm additions on soil carbon transformations [i.e., C assimilated as microbial biomass (Cmic and Cmic/Corg ratio), stored as oxidizable carbon (i.e., salt extractable organic carbon (SEOC)), and released as CO2 and dissolved organic carbon (DOC)] in a mesocosm experiment
Earthworms added to the +EW experimental units significantly increased the population sizes naturally present in these soils (p < 0.0001), resulting in the +EW treatments having 3.9 ± 0.25 individuals on average, when compared to those recorded in the control ones (0.87 ± 0.20 individuals) and this difference was observed in all three management treatments throughout the whole duration of the incubation period (Figure 1)
Agricultural management practices had a significant effect on all variables investigated with the exception of Cmic, SEOC and the DOC/DON ratio (ANOVA, p < 0.05; Table 2)
Summary
Sustainable agricultural practices, such as integrated (INT) and organic (ORG) farming have been increasingly implemented worldwide to enhance soil organic matter retention (Diacono and Montemurro, 2009), aggregate stability (Bai et al, 2018) and soil biodiversity (Ponge et al, 2013; Henneron et al, 2015) as well as to reduce the negative environmental effects of the more intensive conventional (CONV) agriculture, such as soil erosion (Ordóñez et al, 2007; Verhulst et al, 2010), nutrient losses by leaching or run-off (Riley et al, 2001), greenhouse emissions (Bos et al, 2014), as well as to prevent soil acidification (Otero et al, 2008).In Spain, the cultivation of the most commercialized cultivar (Hayward) of the green-fleshed kiwifruit (Actinidia chinensis var. deliciosa) is concentrated in the Galician region (NW Spain) by accounting for almost 60% of the total national production (MAPA, 2019) and representing an important income for rural communities. Deliciosa) is concentrated in the Galician region (NW Spain) by accounting for almost 60% of the total national production (MAPA, 2019) and representing an important income for rural communities. In this region, kiwifruit is mainly produced using conventional agriculture (CONV) that is characterized by the extensive use of agrochemicals, including pesticides, herbicides, plant growth regulators and mineral fertilizers. The number of ORG orchards have increased in the region in recent years, farmers are not fully embracing these sustainable practices due to their lower profitability in terms of crop yields (Seufert et al, 2012). Farming systems need to produce more food in a more sustainable way, and to increase biodiversity and become carbon neutral (European Green Deal: COM/2019/640 final)
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