Microbial respiration in a temperate sown grassland grazed by sheep.
Abstract
Prediction models were developed for calculating microbial respiration of surface residues and soil (0-15 cm) for a sown temperate pasture grazed at 10, 20 or 30 sheep/ha. Relationships were developed to adjust profile samples for respiration of roots and rhizomes at varying soil moistures and temp. Seasonal relationships between microbial respiration and soil moisture, expressed as the ratio of water to dry soil, were rectilinear over a range of 0.10-0.35 (field capacity). Peak respiration values (ml O2 at STP/g soil DM) at opt. moisture were 0.031 (winter 5 deg C), 0.047 (spring 15 deg ), 0.170 (summer 25 deg ) and 0.127 (autumn 15 deg ). Respirations of mixed roots from the 20 sheep/ha stocking treatment were 5.03, 12.37, 16.52 and 8.91 (ml O2 at STP/g DM) for the 4 seasons, winter to autumn inclusive. Values for rhizomes of Phalaris aquatica ranged from 35 to 60% of the seasonal values for mixed roots. Root and rhizome respirations were reduced at low soil moisture levels. The vertical distribution (0-15 cm) of respiration activity in autumn did not differ between stocking levels of 10, 20 and 30 sheep/ha; 71% of activity occurred in the litter-excreta-soil (0-5 cm) zone. Differences in respiration between the stocking treatments were small. Respiration was increased substantially in the night camping areas of the 30 sheep/ha site and was a more sensitive indicator of excretal distribution than the amount of sheep faeces/unit area. 2 prediction models were used to calculate microbial energy expenditure, for 48-h intervals, using 3 yr of field data for soil moisture and temp. Their outputs were influenced strongly by soil moisture status. Integrated annual values for respiration ranged from 232 to 316 GJ/ha (Model 1) and 264 to 359 GJ/ha (Model 2) for the 3 yr. An annual energy budget was completed for microorganisms, sheep and invertebrates in the 20 sheep/ha site. The budget compares favourably with measurements of net primary production made in the same experimental area.