Hubert Boizard

Cumulative effects of cropping systems on the structure of the tilled layer in northern France

Abstract A field experiment was initiated in 1989 in northern France to evaluate the cumulative effects of cropping systems on the structure of the tilled layer of a loamy soil. Three cropping systems involving different crop rotations and cultivations (early or late sowing, early or late harvesting) were compared. The soil structure was evaluated after each winter wheat sowing by a morphological analysis of the ploughed layer. We determined the proportion of highly compacted zones, i.e. the zones with a massive structure and no visible macropores in the soil profile. These zones, defined as Δ zones, result from severe anthropogenic compaction. The creation of Δ zones depended largely on the soil moisture at the time of field operations and the characteristics of the machinery used. Maximum compaction occurred during harvesting in wet conditions because of high axle loads. In contrast, little compaction was produced by seedbed preparation, which involved lower axle loads and wide tyres. Consequently, changes in soil structure depended to a large extent on the cropping system. However, the proportion of Δ areas was not stable, but fluctuated greatly from one year to the next. Δ zones could quickly disappear from the ploughed layer. We, therefore, detected no irreversible effects on the structure of the ploughed layer, even for the cropping system that produced the highest annual risk of soil compaction. The combined effects of tillage and climate led to fewer compacted zones in the surface layer. Because this layer was mixed with deeper layers during soil inversion at the next ploughing, this contributed to fewer Δ zones in the whole layer. The loss of Δ zones over the whole cultivated layer could not be explained by this effect alone and the reduction in soil compaction was probably due to the combined effects of loosening by mouldboard ploughing and climatic and soil fauna activities in the ploughed layer below the seedbed. The initial soil structure had a major effect on seedbed fragmentation. When preparing seedbeds in the autumn, the proportion of remaining clods depended greatly on the initial state of the ploughed layer even when using a power rotary harrow. In spring, the number of remaining clods was still dependent on the initial compaction, but they were fewer and no differences were observed between rotary and combination harrows.

Implications of productivity and nutrient requirements on greenhouse gas balance of annual and perennial bioenergy crops

Biomass from dedicated crops is expected to contribute significantly to the replacement of fossil resources. However, sustainable bioenergy cropping systems must provide high biomass production and low environmental impacts. This study aimed at quantifying biomass production, nutrient removal, expected ethanol production, and greenhouse gas (GHG) balance of six bioenergy crops: Miscanthus × giganteus, switchgrass, fescue, alfalfa, triticale, and fiber sorghum. Biomass production and N, P, K balances (input‐output) were measured during 4 years in a long‐term experiment, which included two nitrogen fertilization treatments. These results were used to calculate a posteriori ‘optimized’ fertilization practices, which would ensure a sustainable production with a nil balance of nutrients. A modified version of the cost/benefit approach proposed by Crutzen et al. (2008), comparing the GHG emissions resulting from N‐P‐K fertilization of bioenergy crops and the GHG emissions saved by replacing fossil fuel, was applied to these ‘optimized’ situations. Biomass production varied among crops between 10.0 (fescue) and 26.9 t DM ha−1 yr−1 (miscanthus harvested early) and the expected ethanol production between 1.3 (alfalfa) and 6.1 t ha−1 yr−1 (miscanthus harvested early). The cost/benefit ratio ranged from 0.10 (miscanthus harvested late) to 0.71 (fescue); it was closely correlated with the N/C ratio of the harvested biomass, except for alfalfa. The amount of saved CO2 emissions varied from 1.0 (fescue) to 8.6 t CO2eq ha−1 yr−1 (miscanthus harvested early or late). Due to its high biomass production, miscanthus was able to combine a high production of ethanol and a large saving of CO2 emissions. Miscanthus and switchgrass harvested late gave the best compromise between low N‐P‐K requirements, high GHG saving per unit of biomass, and high productivity per hectare.