Effects of Compaction and Organic Amendments on Soil Hydraulic Characteristics, Plant Growth Performance and Greenhouse Gas Emission
Compaction alters the soil structure by crushing aggregates and decreasing the number of macro pores resulting in reduced water and air permeability. Compaction can take place on or below the surface of soils. Below-surface compaction can vary from a few millimeters to meters in thickness, and is a crucial factor in determining soil hydraulic characteristics. The application of organic amendments to agricultural soils can interrelate with soil compaction. Both compaction and organic amendments influence soil water movements, plant growth, and terrestrial greenhouse gas (GHG) emissions. There is no sufficient evidence on the effects of surface and subsurface compacted layers on soil hydraulic properties and, how compaction might interfere with the impacts of soil amendments possibly influencing GHG emission and plant growth. This study examined the influence of surface and below-surface compaction and organic amendments on soil hydraulic characteristics, plant growth performance, and GHG emissions, using three experiments. Experiment I aimed to assess the effects of the thickness of the hardpan (THP), (1.7 g cm-3 bulk density) (1, 3, 5 cm) and the depth to the hardpan (DHP) (0, 1, 3, 5 cm) on initial infiltration rate; Ii, unsaturated hydraulic conductivity; KUNSAT, sorptivity; SW and steady - state infiltration rate; ISS. All the thin hardpans were prepared in soil columns at different depths along with a control (no hardpan) and, soil hydraulic characteristics were measured using a mini disk infiltrometer. Hardpans significantly lowered all hydraulic characteristics (p < 0.05), where the surface hardpan revealing >90% decline in Ii, KUNSAT, and SW compared with the control. At DHP1, Ii, KUNSAT, SW, and ISS showed 19–44, 71–95, 43–57, and 50–70% reductions, respectively, at the three thickness levels compared with the control. Experiment confirmed that surface and subsurface hydraulic characteristics can be detrimentally affected by minor level of compaction. Experiment II was conducted to assess the effects of compaction and organic amendments on the growth performance of plants with tap and fibrous root systems for a period of 10 weeks. Cattle manure and biochar made from paddy husk (BCPH), and Casuarina equisetifolia (BCCE) (3% w/w) were amended with soils and compacted (CS: 1.8 g cm-3) and non-compacted (NCS: 1.1 g cm-3) conditions were created in pots. Plant height (weekly), fresh and dry weights of shoots and roots (at 10 weeks) were measured. Compaction significantly lowered (p < 0.05) the plant height, shoot and root biomass of both plants. Organic amendments significantly increased the shoot and root biomass of both plants under NCS conditions (p < 0.05), while BCPH and BCCE showed the most beneficial effects. The effects of soil amendments were less pronounced in CS. In Experiment III, the effects of AAC (50 g kg-1) on CO2 and N2O emissions from compacted and non-compacted paddy soils were examined at 60 and 100% water holding capacity (WHC) levels over 21 days. Both CO2 and N2O emissions were significantly lower at 100% than at 60% WHC (p < 0.05). AAC significantly (p < 0.05) decreased N2O emission during the whole incubation period, and CO2 only at the early stages (~7 d). Compaction lowered the CO2 (p < 0.05) and increased N2O (p < 0.05) emissions, emphasizing the influence of soil physical conditions on GHG emissions. Overall, it can be concluded that soil compaction and soil amendments significantly influence surface and subsurface soil hydraulic characteristics, plant growth performance, and GHG emission.
