Impact of reduced tillage on greenhouse gas emissions and soil carbon stocks in an organic grass-clover ley - winter wheat cropping sequence

Abstract

Organic reduced tillage aims to combine the environmental benefits of organic farming and conservation tillage to increase sustainability and soil quality. In temperate climates, there is currently no knowledge about its impact on greenhouse gas emissions and only little information about soil organic carbon (SOC) stocks in these management systems. We therefore monitored nitrous oxide (N₂O) and methane (CH₄) fluxes besides SOC stocks for two years in a grass-clover ley - winter wheat - cover crop sequence. The monitoring was undertaken in an organically managed long-term tillage trial on a clay rich soil in Switzerland. Reduced tillage (RT) was compared with ploughing (conventional tillage, CT) in interaction with two fertilisation systems, cattle slurry alone (SL) versus cattle manure compost and slurry (MC). Median N₂O and CH₄ flux rates were 13 μg N₂O-N m^-2 h^-1 and -2 μg CH₄C m^-2 h^-1, respectively, with no treatment effects. N₂O fluxes correlated positively with nitrate contents, soil temperature, water filled pore space and dissolved organic carbon and negatively with ammonium contents in soil. Pulse emissions after tillage operations and slurry application dominated cumulative gas emissions. N₂O emissions after tillage operations correlated with SOC contents and collinearly to microbial biomass. There was no tillage system impact on cumulative N₂O emissions in the grass-clover (0.8-0.9 kg N₂O-N ha^-1, 369 days) and winter wheat (2.1-3.0 kg N₂O-N ha^-1, 296 days) cropping seasons, with a tendency towards higher emissions in MC than SL in winter wheat. Including a tillage induced peak after wheat harvest, a full two year data set showed increased cumulative N₂O emissions in RT than CT and in MC than SL. There was no clear treatment influence on cumulative CH₄ uptake. Topsoil SOC accumulation (0-0.1 m) was still ongoing. SOC stocks were more stratified in RT than CT and in MC than SL. Total SOC stocks (0-0.5 m) were higher in RT than CT in SL and similar in MC. Maximum relative SOC stock difference accounted for +8.1 Mg C ha^-1 in RT-MC compared to CT-SL after 13 years which dominated over the relative increase in greenhouse gas emissions. Under these site conditions, organic reduced tillage and manure compost application seems to be a viable greenhouse gas mitigation strategy as long as SOC is sequestered.

Keywords: Grass-clover; Nitrous oxide; Organic farming; Reduced tillage; Soil organic carbon stocks; Wheat.

Fig. 1

N₂O and CH₄ fluxes, soil (0–0.2 m) and environmental parameters during a two year monitoring in a grass-clover – winter wheat – cover crop (CC) sequence. Means (±SE, n = 8) of gas fluxes besides mineral nitrogen (Nmin, nitrate + ammonium) and dissolved organic carbon (DOC) contents, that were sampled treatment wise are displayed per treatment with CT – ploughing, RT – reduced tillage, SL – slurry and MC – manure compost. Soil water filled pore space (WFPS) is shown as mean (±SE, n = 4) across all treatments. Soil temperature in 0.1 m depth was recorded once during sampling. Daily precipitation and mean daily air temperature derive from the weather station.

Fig. 2

Management induced cumulative N₂O emissions per cropping season (GC ley – grass-clover, WW – winter wheat, CC – cover crop) and total cumulative emissions of the two year monitoring period. Management induced emissions are assigned to emissions after slurry application and tillage operations (periods see Table S1, Supplement material). Baseline emissions refer to remaining emissions in the respective cropping period . Total two years N₂O emissions are represented by the entire bar and by values displayed on top. Mean cumulative N₂O emissions (n = 8) are displayed per treatment with CT – ploughing, RT – reduced tillage, SL – slurry and MC – manure compost. Significant tillage/fertiliser system effects on total N₂O-N emissions and emissions in each period are shown on the right hand side (ANCOVA, F-test, Level of significance: (*)p < 0.1, *p < 0.05, ** p < 0.01, ns = not significant).

Fig. 3

Development of soil organic carbon (SOC) concentrations from trial start in 2002 to 2015 in three soil layers (0–0.1, 0.1–0.2, 0.2–0.5 m). Means (±SE, n = 4) are displayed per treatment with CT – ploughing, RT – reduced tillage, SL – slurry and MC – manure compost.