Aggregate Size Distribution Indices Influenced by Different Types/Managements of Plant Residues under Field Conditions

1- INTRODUCTION Nowadays, environmental aspects of soil erosion are specifically considered by researchers. In this regard, secondary (aggregate) particle size distribution (SPSD) is so important in different issues such as nutrient loss, pollutant transport and carbon cycle. Aggregate size distribution is known as the representative of soil structure, which is by itself regarded as a dynamic soil feature. The type and application method of plant residues have a significant influence on the aggregate size distribution. Therefore, the accurate utilization of plant residue management after the harvesting procedure is of crucial environmental issues. However, farmers in some parts of the country, burn plant residues remaining after harvesting; this leads to the destruction of soil structure, greenhouse gas emissions and environmental damages. Therefore, the purpose of the current study was to investigate the effects of the type and different managements of plant residues on the aggregate size distribution indices. 2- THEORETICAL FRAMEWORK Several studies around the world and in Iran have been conducted on the impact of different management methods of crop residues on the aggregate size distribution. In this context, it has been found that through mixing and/or keeping crop residue after harvesting, the mean weight diameter (MWD) of the aggregates increased significantly in both wet and dry cases as compared with the control. On the other hand, due to the positive effects of these two methods of management on the structural stability compared to burning, the formation and development of the surface crust decreases. Compared to the favorable effects of mixing and keeping plant residues in farm on aggregate size distribution, some destructive operations such as burning have negative influences. 3- METHODOLOGY To obtain the objectives, a field experiment was done as factorial based on RCBD with three replicates. The first factor was the residue type including barely straw and alfalfa residue, and the second factor was different management practices including 1) incorporating one percent of the plant residues into the soil, 2) incorporating 0.5 percent of the plant residues into the soil, 3) surface retention­­ of plant­ residues, 4) burning of plant residues and 5) control. After nine months, the aggregate size distribution indices including the mean weight diameter (MWD) and the geometric mean diameter (GMD) both at two cases of wet and dry and also water stable aggregates (WSA) and dry stable aggregates (DSA) were measured. 4- RESULTS Results of the variance analysis showed that the influence of plant residue type was just significant on MWD and GMD in wet case, whereas, the effect of plant residue management was significant on all the studied indices. This indicates that the management of plant residues is more important than the type of residues. Also, barley straw was more effective than alfalfa residue in improving the aggregate size indices, which was attributed to higher content of carbon and greater C:N ratio in the composition of barley straw. Also, in all cases, mixing one percent of plant residues with soil showed greater role in improving the soil structure than the other application methods of residues. The incorporation of one percent barely straw led to the highest influence on the improvement of aggregate size distribution, so that due to the application of this treatment as compared with control, MWD and GMD increased respectively by 87% and 68.6% (under wet conditions) and by 33.6% and 21% (under dry conditions), while WSA and DSA were enhanced by 86.4% and 23.7%, respectively. Moreover, the surface application of residues resulted in aggregate size improvement, however, its effectiveness was less than the incorporation method. In contrast, the burning of residues reduced almost all the indices compared to the control. 5- CONCLUSIONS & SUGGESTIONS In general, the priority of residue application methods regarding the improvement of aggregate size distribution was determined respectively as: burning