Influence of the order in which low and high C/N residues on soil nutrient availability and wheat nutrient uptake

Abstract

It is well-known that the C/N ratio of plant residues can influence soil nutrient availability, but the effect of repeated addition of plant residues with different C/N ratio is less explored. In previous studies, we showed that nutrient availability and soil respiration after the second residue addition is influenced not only by the C/N ratio of that residue, but also by the C/N ratio of the previously added residue. These experiments were carried out without plants and it was unclear how the legacy effect would influence plant growth and nutrient uptake. The aim of this experiment was to assess plant growth, nutrient uptake and soil nutrient availability after the second residue addition with different length of time between the first and second residue addition where the first and second residue had the same or a different C/N ratio. High (H) or low C/N (L) residue was added at the start of the experiment, the second residue with either the same or a different C/N ratio was added on days 7, 14, 21 or 28 with a total residue addition of 20 g kg^-1 giving four residue treatments: HH, LL, LH and HL. Wheat was planted immediately after the second residue addition and grown for 28 days. N and P availability were measured on days 7, 14, 21 and 28 and at plant harvest. Soil N and P availability after the second residue addition were in the order HH<LH<HL<LL. Wheat biomass generally did not differ between LL, HL and LH, but wheat in HL and LH had a lower shoot/root ratio than in LL suggesting that in HL and LH the plants were able to compensate the lower nutrient availability by increased root growth. In conclusion, the C/N ratio of the previous residue addition influenced nutrient availability after the second residue addition, but plant growth did not differ between HL, LH and L because plants in the former developed a more extensive root system and could therefore access the nutrients released during decomposition of L even in treatments where both H and L were present in the soil.