Resumen de Amendment with high and low C/N residues- Influence of rate, order and frequency
Thi Hoang Ha Truong, Petra Marschner
It is unclear if the effect of residue mixes on soil respiration, microbial biomass and nutrient availability over time is influenced by amendment frequency and how the effect differs from that of the same residues added sequentially. There were six treatments differing in number of amendments and order in which residues were added, total amendment rate in all treatments was 20 g kg-1 with 10 g kg-1 of each high (H) and low (L) C/N residue. In treatment names, order of letters indicates order of residues, e.g. HL is H followed by L. In treatments with two amendments, residues were added on days 0 and 20 at 10 g kg-1: 10-LH, 10-HL or a 1:1 mixture of L and H added twice [10-(HL)x2]. In treatments with four amendments, residues were added on days 0, 10, 20 and 30 at 5 g kg-1: 5-HLHL, 5-LHLH, and 5(HL)x4. In 5-HLHL, microbial biomass N (MBN) increased only after the first L addition although available N increased after both additions. Differences between measured and expected value depended on residue addition frequency and parameter. In 10-(HL)x2, MBN, microbial biomass P and C (MBP and MBC) were greater than expected and this was accompanied by lower than expected available N and P. In 5-(HL)x4 on the other hand, the difference between measured and expected MBN and available N changed over time, possibly because the proximity of microbes decomposing different residues changes. The study showed that with repeated addition of H and L, N availability and MBN are influenced by residue rate and order.
