Diversity of Crop Residue Mulches in Conservation Agriculture and Their Role in Water Retention.

Conservation tillage and plant residue mulches play a vital role in moisture conservation, water infiltration, maintaining temperature and stabilizing soil structure. In semi-arid regions, mulching has been shown to be effective in reducing the risk of crop failure at field level due to better capture and use of rainfall. The capacity of the mulch to capture rain water and to retard the evaporation depends on many factors like type of mulch, biomass and rate of soil cover and physical and chemical properties. According to Findeling et al. (2007) and Coppens et al. (2007), maximum water retention of mulch particles is an important parameter in modeling the decomposition of mulch, controlling the exchanges between soil, mulch and the atmosphere and influencing the activity of soil microorganisms.

In this context, the specific objectives of this study were to characterize different types of mulches present in CA situations according to crop species (with pure or mixed crops), their biomass characteristics (amount, thickness) and their water retention properties. The experimental sites and farm fields studied were located in two regions of France (organic farming in Rhônes-Alpes and intensive agriculture in Champagne-Ardenne), in the Cerrados area of Brazil and in Madagascar.

We surveyed the diversity of agricultural situations, e.g. the types of crops involved, the scenarii of crop rotations and the type and characteristics of residue mulches. The water retention properties were assessed on maize, rice, wheat and canola residues. It was done by using stems of 5cm particle size as an experimental model representing field mulch particles. Firstly the maximal and residual water retention were determined by immersing particles in deionized water for a given time (set up according to the nature of the residue) then drying them until constant weight at 120°C. Water retention was also measured at different pressure heads (0 to 4.2 pF) using Richard’s method.

In the situations of northern France (Nouricia network) , there were mainly two situations, mulches of single crops (Maize, wheat, canola etc.) or mulches of mixed crops formed by a main crop + a cover crop (e.g. Wheat + grass/legume mixed). In other situations of the project, the diversity of situations was much greater, e.g association of several crops (e.g. maize + signal grass + Cajan pea + Crotalaria at Madagascar) or in rotation (e.g. Rice + Cajan pea).  The biomass of mulch depended on the type of residues, climate and practices adopted for production, varying from 2 to 14 t/ha. In the situation explored (sunflower, canola, wheat etc.), the thickness of mulches varies from 7 to 13 mm.

The maximum absorbed water content of residue varied between plant type (i.e for maize, canola, wheat and rice the water contents were 3.0, 4.0, 2.7 and 2.4 g H₂0 g^-1 residue, respectively). For a given residue, maximum water content varied according to decomposition (i.e. for maize stem the maximal water contents were 3.0, 8.0 and 13.0 g H₂0 g^-1 residue after 0, 49 and 105 days of decomposition at 25°C, respectively). The water retention was closely related to the plant biochemical composition, expressed as the ratio of lignin to holocellulose plus lignin content (LCI).