Document: DES-3-59-85

Root effects on soil carbon and nitrogen cycling in a Pinus radiata plantation.

ROSS, D.J.*, N.A.SCOTT, K.R.TATE, N.J.RODDA and J.A.TOWNSEND

Landcare Research, Palmerston North, NEW ZEALAND ¹

Abstract:
While many studies have shown that removing root inputs alters soil nitrogen (N) cycling (e.g. nitrification), no consistent mechanistic explanation has been identified. Trenching experiments have also been used to quantify belowground carbon (C) allocation without always considering trenching effects on soil moisture. We used replicated (3) trenched and control plots in a 25-year-old Pinus radiata plantation to examine root C allocation and root effects on soil C and N cycling. Experimental plots (4 by 4 m) were trenched to 1 m depth (water table) in June 1996. In situ CO₂ production, soil moisture, soil temperature, and net N mineralization (resin bags) were subsequently measured over two years. After two years, soil samples (00.1, 0.10.2 m) were removed for laboratory measurements of key biochemical properties (e.g. soil CO₂ production, microbial C and N, extractable C, and net and gross N mineralization and nitrification. In situ soil respiration rates averaged about 25% lower in the trenched plots, with the largest differences occurring when soils were moist. Laboratory-based CO₂ production under standardized conditions was also lower in soil from trenched plots, with the largest difference in the 0-0.1 m layer (0.49 v. 0.28 mg kg^-1 h^-1, P<0.01), and correlated closely with extractable C concentrations. We used the laboratory and field results to estimate the root contribution to soil respiration, which was about 55%. In situ net N mineralization was 40-fold higher and net nitrification was 200-fold higher in the trenched plots. However, during laboratory incubations, net N mineralization in mineral soil from trenched and untrenched plots did not differ (P<0.1) except in the FH layer, where it was two-fold higher in the trenched plot between 014 days. Net nitrification was always higher in soils from the trenched plots in both laboratory and in situ incubations. Gross nitrification occurred readily in soils from both trenched and untrenched plots, but was consistently higher in soil from the trenched plots. Nitrate immobilization was also higher in the trenched plots, suggesting that high nitrification associated with removal of root inputs resulted from nitrification inhibition in the control plots.

Keywords: soil respiration, root respiration, nitrification, trenching, nitrogen

This abstract is being presented at: 3:30 PM in session:
ROOT PROCESSES