Soil organic N dynamics and stand quality in Pinus radiata pinewoods of the temperate humid region.

Abstract

Soil organic-N dynamics, its controlling factors and its relationships with stand quality were studied in the 0-15 cm soil layer of 24 pinewoods with contrasting age, productivity and parent material (granite; acid schists), searching for N variables useful to predict stand growth and site quality. No significant differences were found between young and old stands for any of the N variables considered, nor two- or three-order interactions among stand age, site quality and parent material. The soil total-N content, which was correlated positively with the Al oxides content [a soil organic matter (SOM) stabilizing agent], did not vary significantly according to parent material, but it was lower (P # 0.02) in stands with high than with low site index (2.68 ± 1.11 and 3.97 ± 1.13 g N kg-1 soil, respectively). The soil δ 15N ranged from +3.5 to +6.5 d, without significant differences among stand groups, and it was negatively correlated with water holding capacity, exchangeable bases, Al oxides and N content, suggesting that: i) N losses by NO3 - leaching are the most important controlling factor of δ 15N in these temperate humid region soils; and ii) soil N richness is related with limited N losses, which discriminate against 15N. At any incubation time, no significant differences were found in soil inorganic-N content among stand groups (7.78 ± 4.57, 39.33 ± 16.20 and 67.80 ± 26.50 mg N kg-1 soil at 0, 42 and 84 d, respectively). During the incubation, the relative importance of the ammonification decreased and that of the nitrification increased. The net N mineralization rate (NNMR, in % of organic N) was significantly higher in granite than in schists soils at both 42 d (1.24 ± 0.34 and 0.75 ± 0.37%, respectively) and 84 d (2.18 ± 0.56 and 1.53 ± 0.66%, respectively). In high quality pinewoods, the NNMR at 42 d and 84 d (1.16 ± 0.45 and 2.12 ± 0.79%, respectively) were significantly higher than in low quality stands (0.83 ± 0.35 and 1.59 ± 0.45%, respectively). This result, together with those on soil total-N and inorganic-N supply, suggests that soil N dynamics in low and high quality stands is different: in the former there is a bigger N pool with a slower turnover, whereas in the latter there is a smaller N pool with a faster turnover, both factors being nearly compensated, making similar the soil available N supply. After 42 and 84 d of incubation, the NNMR and the nitrification rate were higher in the coarse textured soils, likely due to the low physical and chemical protection of their SOM; both rates were positively correlated with available P, exchangeable K+ and CEC base saturation, suggesting strong relationships among the availabilities of the main plant nutrients, and they increased with SOM quality (low C-to-N ratio). The strong negative correlation of site index with soil total-N (r= - 0.707; P # 0.005), and its positive correlations with NNMR after 42 and 84 d of incubation, suggested that site quality and potential productivity are closely related to soil organic-N dynamics. Half of the site index variation in the stands studied could be predicted with a cheap and easy analysis of soil N content, the prediction being slightly improved if soil δ 15N is included and, more significantly, by including N mineralization measurements.