Soil moisture and soil strength interaction impacts on root distribution and dry matter production in rainfed lowland rice field in the Philippines

Rainfed lowland rice productivity is largely determined by the plasticity in root system development, which is affected by soil moisture fluctuations and hardpan, but empirical evidence is still lacking. This study aimed to characterize soil environment in rainfed lowland rice areas through constant and intensive measurements and observations of soil moisture and hardness distribution in soil depths and coupled with root distribution. Field experiments were conducted in 2001 wet (WS) and 2012 dry (DS) seasons in farmers' field in Nueva Ecija, Pangasinan provinces and three (uppermost, mid and lowermost) toposequences in Isabela province. Several genotypes including drought resistant strains developed for drought tolerance by PhilRice and IR64 introgression lines were used together with IR64 and NSIC Rc9 checks. Soil moisture contents (SMC) and penetration resistance (PR) were monitored during the experiments. After harvesting, shoot biomass, grain yield and root length were measured. Soil moisture fluctuations differed among sites but were more pronounced in the dry season in terms of frequency and intensity of drought stress. In 2011 WS, penetration resistance becomes stronger at the hardpan layer (10-20 cm depth) with decreasing soil moisture in both the uppermost and mid toposequences in Isabela sites only. In 2012 DS, penetration resistance had relationship with changes in SMC at 20-30 cm soil depths in all toposequences in Isabela, at 10-20 cm depth in Nueva Ecija but no relationships were observed in Pangasinan. Genotypic differences in root distribution varied among sites. In both seasons, the lateral root length at 20-30 cm soil depths in Isabela and Nueva Ecija and at 10-20 cm depth in Pangasinan had positive contributions to total biomass production. This implied that the genotype that showed greater dry matter production under rainfed conditions may have exhibited the root plasticity in response to the reduction of soil hardness with increasing soil moistures in the hardpan.