Cyril Girardin

Fourteen years of evidence for positive effects of conservation agriculture and organic farming on soil life

Conventional agriculture strongly alters soil quality due to industrial practices that often have negative effects on soil life. Alternative systems such as conservation agriculture and organic farming could restore better conditions for soil organisms. Improving soil life should in turn improve soil quality and farming sustainability. Here, we have compared for the first time the long-term effects of conservation agriculture, organic farming, and conventional agriculture on major soil organisms such as microbes, nematofauna, and macrofauna. We have also analyzed functional groups. Soils were sampled at the 14-year-old experimental site of La Cage, near Versailles, France. The microbial community was analyzed using molecular biology techniques. Nematofauna and macrofauna were analyzed and classified into functional groups. Our results show that both conservation and organic systems increased the abundance and biomass of all soil organisms, except predaceous nematodes. For example, macrofauna increased from 100 to 2,500xa0%, nematodes from 100 to 700xa0%, and microorganisms from 30 to 70xa0%. Conservation agriculture showed a higher overall improvement than organic farming. Conservation agriculture increased the number of many organisms such as bacteria, fungi, anecic earthworms, and phytophagous and rhizophagous arthropods. Organic farming improved mainly the bacterial pathway of the soil food web and endogeic and anecic earthworms. Overall, our study shows that long-term, no-tillage, and cover crops are better for soil biota than periodic legume green manures, pesticides, and mineral fertilizers.

Biochar mineralization and priming effect on SOM decomposition in two European short rotation coppices.

As studies on biochar stability in field conditions are very scarce, the carbon sequestration potential of biochar application to agricultural soils remains uncertain. This study assessed the stability of biochar in field conditions, the effect of plant roots on biochar stability and the effect of biochar on original soil organic matter (SOM) decomposition in two (Italy and United Kingdom) short rotation coppice systems (SRCs), using continuous soil respiration monitoring and periodic isotopic (δ13CO2) measurements. When root growth was excluded, only 7% and 3% of the biochar carbon added was decomposed after 245 and 164 days in Italy and United Kingdom sites respectively. In the presence of roots, this percentage was increased to 9% and 8%, suggesting a small positive priming effect of roots on biochar decomposition. A decreased decomposition rate of original SOM was observed at both sites after biochar incorporation, suggesting a protective effect of biochar on SOM. This study supports the carbon sequestration potential of biochar and highlights the role of root activity on biochar decomposition, questioning the applicability of laboratory incubation studies to assess biochar stability.

Biochar alters the soil microbiome and soil function: results of next‐generation amplicon sequencing across Europe

Wide‐scale application of biochar to soil has been suggested as a mechanism to offset increases in CO2 emissions through the long‐term sequestration of a carbon rich and inert substance to the soil, but the implications of this for soil diversity and function remain to be determined. Biochar is capable of inducing changes in soil bacterial communities, but the exact impacts of its application are poorly understood. Using three European sites [UK SRC, short rotation coppice, French grassland (FR) and Italian SRF, short rotation forestry (IT)] treated with identical biochar applications, we undertook 16S and ITS amplicon DNA sequencing. In addition, we carried out assessments of community change over time and N and P mobilization in the UK. Significant changes in bacterial and community structure occurred due to treatment, although the nature of the changes varied by site. STAMP differential abundance analysis showed enrichment of Gemmatimonadete and Acidobacteria in UK biochar plots 1 year after application, whilst control plots exhibited enriched Gemmataceae, Isosphaeraceae and Koribacteraceae. Increased mobility of ammonium and phosphates was also detected after 1 year, coupled with a shift from acid to alkaline phosphomonoesterase activity, which may suggest an ecological and functional shift towards a more copiotrophic ecology. Italy also exhibited enrichments, in both the Proteobacteria (driven by an increase in the order Rhizobiales) and the Gemmatimonadetes. No significant change in the abundance of individual taxa was noted in FR, although a small significant change in unweighted UNIFRAC occurred, indicating variation in the identities of taxa present due to treatment. Fungal β diversity was affected by treatment in IT and FR, but was unaffected in UK samples. The effects of time and site were greater than that of biochar application in UK samples. Overall, this report gives a tantalizing view of the soil microbiome at several sites across Europe and suggests that although application of biochar has significant effects on microbial communities, these may be small compared with the highly variable soil microbiome that is found in different soils and changes with time.

Effect of biochar addition on C mineralisation and soil organic matter priming in two subsoil horizons

PurposeThe impact of biochar on subsoil organic carbon mineralisation has never been assessed despite its susceptibility to downward transport after soil amendment. In this study, we analysed the potential mineralisation of biochar and plant material as well as their effect on native soil organic matter (SOM) decomposition in subsoil horizons.Materials and methodsWe used 13C-labelled biochar and plant material to allow disentangling substrate mineralisation and priming effects. The substrates were added to two mountain subsoils under different land use and incubated for 1xa0year under optimum conditions. We analysed for physical parameters and C mineralisation in the two soils. Moreover, microbial communities were assessed by phospholipid fatty acid (PLFA) analyses.Results and discussionOur results indicated contrasting potential C mineralisation of subsoils under different land use probably related to sampling depth, contribution of stabilised organic matter compounds, carbon content as well as quality. In general, very low proportions of biochar were mineralised in the two soils as compared to plant material. The mineralisation of each of the added substrates (biochar and plant material) was slightly, but significantly different in the two soils. Native C mineralisation was much higher after plant material addition than after biochar addition. Subsoil type influenced the kind and magnitude of priming effects for both added substrates.ConclusionsBiochar mineralisation and its priming effects in subsoil are small as compared to uncharred plant litter. We suggest that substrate mineralization and priming effects induced on subsoil organic matter are dependent on the composition of the added substrate, as well as soil parameters rather than microbial community characteristics.

Changes in δ13C of dark respired CO2 and organic matter of different organs during early ontogeny in peanut plants

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48u2005%) compared with bean seeds (1.5u2005%), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became 13C-enriched reaching −21.5u2005‰, while root-respired CO2 became 13C-depleted reaching around −31u2005‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly 13C-enriched, and its 13C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrate. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the 13C-enriched cotyledon-respired CO2 is discussed.

Environmental factors controlling soil organic carbon stability in French forest soils

AimsIn temperate forests, soils contain a large part of the ecosystem carbon that can be partially lost or gained upon global change. Our aim was to identify the factors controlling soil organic carbon (SOC) stability in a wide part of French forests.MethodsUsing a set of soils from 53 French forest sites, we assessed the effects of depth (up to 1xa0m), soil class (dystric Cambisol; eutric Cambisol; entic Podzol), vegetation types (deciduous; coniferous) and climate (continental influence; oceanic influence; mountainous influence) on SOC stability using indicators derived from laboratory incubation, physical fractionation and thermal analysis.ResultsLabile SOC pools decreased while stable SOC pool increased with depth. Soil class also significantly influenced SOC stability. Eutric Cambisols had less labile SOC in surface layers but had more labile SOC at depth (> 40xa0cm) than the other soil classes. Vegetation influenced thermal indicators of SOC pools mainly in topsoils (0–10xa0cm). Mountainous climate forest soils had a low thermal SOC stability.ConclusionsOn top of the expected effect of depth, this study also illustrates the noticeable effect of soil class on SOC stability. It suggests that environmental variables should be included when mapping climate regulation soil service.

Effect of in-situ aged and fresh biochar on soil hydraulic conditions and microbial C use under drought conditions

Biochar (BC) amendments may be suitable to increase the ecosystems resistance to drought due to their positive effects on soil water retention and availability. We investigated the effect of BC in situ ageing on water availability and microbial parameters of a grassland soil. We used soil containing 13C labeled BC and determined its water holding capacity, microbial biomass and activity during a 3 months incubation under optimum and drought conditions. Our incubation experiment comprised three treatments: soil without BC (Control), soil containing aged BC (BCaged) and soil containing fresh BC (BCfresh), under optimum soil water (pF 1.8) and drought conditions (pF 3.5). Under optimum water as well as drought conditions, soils containing BC showed higher soil organic carbon (SOC) mineralization as compared to control soil. Moreover, BC effects on the soil water regime increase upon in situ aging. Native SOC mineralization increased most for soils containing BCaged. The BCaged led to improved C use under drought as compared to the other treatments. We conclude that BC addition to soils can ameliorate their water regime, especially under drought conditions. This beneficial effect of BC increases upon its aging, which also improved native substrate availability.