Decomposition of 13C and 15N labelled plant residue materials in two different soil types and its impact on soil carbon, nitrogen, aggregate stability, and aggregate formation

Increasing soil organic matter (SOM) is a major factor in overcoming soil degradation. An incubation experiment using 2 soil types (Red Clay and Black Earth) and 2 different rotations, a clover (Trifolium subterraneum)/cereal rotation and a long fallow/cereal rotation, from a long-term crop rotation trial located at Tamworth, NSW, Australia was conducted to investigate the decomposition of 3 different plant materials, medic (Medicago truncatula) (C : N = 13), rice straw (Oryza sativa) (C : N = 25) and flemingia leaf (Flemingia macrophylla) (C : N = 13), labelled with 13C and 15N. A control treatment with no added residue was also included. The impact of the residue decomposition on total organic carbon, labile carbon, total nitrogen, aggregate stability and the formation of large macro-aggregates from smaller macro-aggregates were studied. Total C (CT), stable carbon isotope composition (δ13C), total N (NT), and %15N excess were measured by catalytic combustion and an isotope ratio mass spectrophotometer, while labile C (CL) was determined by oxidation with KMnO4. Aggregate stability [mean weight diameter (MWD)] was determined by immersion wet sieving. Correlations of C fractions with MWD were also investigated. The location of the newly added plant residue materials within soil aggregates was studied using a soil aggregate eroding machine. Loss of C from the added plant residues was highest for the medic and lowest for the flemingia, while the rice straw initially lost C at a slower rate but by 200 days was equal to the medic. The medic treatment was the only residue to lose N by gaseous loss during the experiment and it was all lost during the first 10 days. In both soils, the addition of residues increased CT and CL compared with the control treatment, with flemingia showing the greatest increase. Factors other than their C : N ratio were clearly determining C turnover. Addition of medic residues resulted in a rapid increase in MWD in both soils in the first 10 days compared with that at the commencement of the experiment. However, this was not maintained for the 200 days by which time MWD had decreased, but it was still greater than the starting point. By contrast, the addition of flemingia leaf exhibited a slower but more sustained increase to have the highest MWD at 200 days, equal to that of the medic treatment at 10 days. There was a positive correlation of CL with MWD at 200 days for both soils. Results from the soil aggregate eroding machine showed that a higher percentage of CT was derived from added plant residues in the outer one-third of the soil aggregates than in the inner two-thirds, with the greatest difference being for the flemingia treatment. There was no difference between different residue materials in the amount of CT derived from the added residues in the inner parts of soil aggregates. These results showed that soil macro-aggregates were forming around a central old aggregate by binding of smaller aggregates to it, with products formed as a result of the breakdown of plant residues binding them together. From the results obtained, and those of other researchers, a concept of macro-aggregate formation under different agricultural systems is proposed.