Influence of Plant Litter and Animal Excreta on Leaching of Dissolved Organic Nitrogen and Carbon in Pastoral Soil.
Anwar Ghani, Moira Dexter, Martin Kear, Stuart Lindsey, and Stewart Ledgard. AgResearch, Ruakura Research Centre, East Street, Hamilton, New Zealand
Dissolved Organic Nitrogen (DON) and Carbon (DOC) are important constituents of the soil solution. Loss of DOC through leaching is being implicated in the loss of organic matter from soils and loss of DON from surface soils to groundwater and streams is of concern from a N balance and ecological viewpoint. Most of the published studies on leaching of DON and DOC have been reported from forestry and cropping soils. There is very little information on the loss of DOC and DON under grazed pastures, which is the dominant land use in New Zealand agriculture. The quantity of DON and DOC leached from grazed pastoral soils will depend on inputs from sources including animal (urine and faeces), pasture (grass and clover) residues, fertiliser and native organic matter. We examined effects of these inputs on the leaching of DON and DOC from soils using intact soil cores (23 cm diameter x 25 cm depth) containing resident perennial grass/clover pasture. To compare the effects of different land uses on the leaching of DON and DOC, we also collected intact cores from adjacent native forest and pine plantation forestry sites located on the same soil type. Thirty five soil cores were collected from a pastoral site located in the Lake Taupo catchment area, North Island, New Zealand. The following treatments were applied on these cores: control, urine patch (equivalent of 600 kg N ha-1), dung patch (1 kg fresh cow dung/core), urea (100 kg N ha-1), ryegrass litter (equivalent to 1000 kg DM ha-1), clover litter (equivalent to 1000 kg DM ha-1), glyphosate (to induce decay of root tissues). Treatments were replicated 5 times. These lysimeters were placed in a growth chamber (at 200C, 16 hr days and 8 hr nights) for five months and leached with 1500 ml reverse-osmosis water at two weekly intervals. The leachates from each soil core were collected, weighed and analysed for nitrate, ammonium, total N and organic and inorganic C. Herbage from the intact cores was harvested every 3-4 weeks. At the time of collecting the intact cores, adjacent soil samples were collected from 0-7.5, 7.5-15 and 15-30 cm depths and characterised for extractable DON and DOC, mineralisable N, microbial biomass-C and -N, hot-water extractable C, total C and N. After 5 months, the intact cores were destructively sampled at 0-7.5, 7.5-15 and 15-25 cm depths and analysed for the same soil characteristics. This enabled us to determine if treatments had any effect on the movement of soluble organic matter. The root senescence/decomposition caused by application of glyphosate showed significantly (P<0.001) greater amounts (15.2 kg C ha-1) of leaching of DOC from the pastoral soil than any other treatment. The second highest DOC leaching (10.5 kg C ha-1) occurred from the urine treatment. There was no significant effect of the dung, grass or clover litter or fertiliser N application on the leaching of DOC which was 5.9-7.4 kg C ha-1. In comparison to pastoral soil, native and plantation forestry sites resulted in a significantly (P<0.001) greater loss of DOC through leaching at 17.1 and 22.3 kg C ha-1, respectively. Loss of DON in the leachates was significantly higher in the urine (31 kg N ha-1) treatment followed by glyphosate treatment (5.2 kg N ha-1). Other treatments showed between 1.5 to 2.5 kg N ha-1 losses of N as DON. Native and forestry soils showed 2 and 2.8 kg ha-1 loss of N as DON respectively. The ratio of DOC:DON in the leachates from native and forestry soils was considerably higher than from pastoral soils. In pastoral soils, this ratio in the control and plant litter treatments ranged between 4.1-4.7 and in dung and glyphosate treatments 2.2 and 2.9 respectively. The urine treatment showed the lowest (0.3) DOC:DON ratio. The large amounts of DON leaching and low DOC:DON ratio suggests that under grazed pastures, urine patches would largely be responsible for most of the N lost as DON. Over 95-99% of the losses of mineral-N occurred as nitrate-N. Again due to highest input of N in the urine treatment, this treatment also showed the greatest leaching of N as nitrate-N (132 kg N ha-1), followed by glyphosate, dung and urea treatments at 21.4, 17.3 and 9.6 kg N ha-1, respectively. The control treatment in the pastoral soil leached only 4.5 kg ha-1 of N as nitrate-N. Our results show that under pastoral land use, urine patches and decomposition of roots can encourage significant losses of C and N in the forms of dissolved organic matter. In contract, dung which can also contribute large quantities of organic N on pastoral soils, contributed very little to leaching losses of DON and DOC. Further research is required to understand the ecological importance of the export of these nutrients beyond the plant rooting depths.