Effect or rice straw and digestate soil incorporation on growth o weedy rice (Oryza sativa) and barnyardgrass (Echinochloa crus-galli)

The recent development of the bioenergy sector has made significant amounts of digestate available, as by-product of anaerobic digestion of biomasses. As several plants have been set up in the Italian rice area, there is a growing interest on using digestate as organic fertilizer for rice cultivation. In this crop, straw is mainly incorporated into the soil, being the main organic input. Alternatively, straw can be removed from the field or burned. The study, conducted during 2014 in Italy, aimed at comparing the effect of two straw management practices (presence or absence of rice straw incorporation) and four different fertilizations (no fertilization, whole digestate, solid digestate, and urea) on the growth of two rice weeds: weedy rice (Oryza sativa L.) and barnyardgrass (Echinochloa crus-galli P. Beauv.). The study was carried out in 15-L plastic pots (with three replications) filled with paddy soil. Before pot filling, the soil was sieved to remove most of the straw residues already present. For the treatments that included straw incorporation, an amount of rice straw equivalent to 7 t ha-1 was added to each pot and mixed with the soil. Digestate or solid digestate was added at a rate of 22 t ha-1 and incorporated in the first centimeters. The pots that received urea were fertilized at a rate equivalent to 100 kg N ha-1. Twenty seeds of O. sativa or E. crus-galli were seeded on the soil surface of each pot; during plant growth two additional top-dress fertilizations with urea were given to all the pots at a rate equivalent to 30 kg N ha-1. Pots were maintained in open field and kept flooded with 3 dry periods of few days. Several times during the season, different growth parameters were assessed: plant height, growth stage, tiller number, flag leaf length, and leaf chlorophyll content (SPAD readings). At harvest, plant biomass, number of culms and panicles and seed biomass were determined. At the last assessment, for both weedy rice (O. sativa) and E. crus-galli, no significant effects of straw incorporation and fertilization were found on plant height, SPAD readings, culm and panicle number. In weedy rice, plant biomass varied according to the fertilization adopted but only in absence of straw incorporation, with the unfertilized pots showing the lowest plant weight (3.6 g plant-1), followed by whole digestate. The highest weight was recorded by both solid digestate and urea with 5 g plant-1. E. crus-galli showed in absence of straw incorporation, the lowest plant biomass in the unfertilized pots (2.7 g plant-1) and the highest in those with urea (5.0 g plant-1). Pots with straw incorporation showed differences in plant biomass only between urea (6.4 g plant-1) and all the other treatments (3.5 g plant-1). Seed biomass varied among fertilization in E. crus-galli only, showing the highest values in pots fertilized with urea. Differences in terms of weed growth were more evident without straw incorporation. Urea determined the highest growth values while digestate application showed intermediate values between mineral and non fertilization.