Effects of Burning and Nutrient Dynamics During the Turnip Crop

The soil pH at the SA01 site was elevated significantly by more than 1 unit (6.770.05) soon after burning and remained near neutral throughout the growth of the turnip crop (Fig. 1). During the same period, soil pH at the CP

Days after burning

Figure 1: Soil pH levels at the CP site (°) and at the SA01 site (•). Vertical bars represent 7 one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

Days after burning

Figure 1: Soil pH levels at the CP site (°) and at the SA01 site (•). Vertical bars represent 7 one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

site stayed in the range 5.4-5.9. The NH4-N content of the topsoil at the SA01 site rose significantly from 297 2.8 mgg 1 before burning to 130719.8 mgg 1 the day after burning (Fig. 2a). After this initial increase, the NH4-N content of the topsoil gradually decreased, reaching pre-burn levels in mid-November (2672.7 mgg1). In contrast, the pre-burn NO3-N content of the topsoil (2573.2 mgg remained unchanged after burning (2473.8mgg 1 Fig. 2b) and then underwent a slow decline, reaching 1472.5 mgg 1 by mid-November. Inorganic N content of the topsoil at the CP site, which was initially lower than that at the pre-burn SA01 site, remained constant during the growth of the turnip crop and then rose, especially NO3-N, in November after the turnips were harvested (Fig. 2b).

At the SA01 site, soluble P content of the topsoil increased 15.6-fold from 16.272.4 mg P2O5 g 1 before burning to 248.2736.5 mg P2O5g~1 the day after burning (Fig. 3a). Soluble P content of the topsoil decreased in mid-October during crop development, subsequently rising back to the immediate post-fire levels (256.0744.2 mg P2O5g_1) in mid-November after the turnip harvest. In contrast, soluble P content of the topsoil at the CP site remained constant at a lower level (16.9-42.1 mg P2O5g_1) throughout the growing season.

Exchangeable cations in the topsoils at the SA01 site increased significantly after burning: 1.970.17 to 4.471.04mmolCg_1 for K, 17.471.51 to 47.0711.68 mmolCg_1 for Ca, and 5.570.30 to 9.171.5 mmolCg_1 for Mg

(b) Days after burning

Figure 2: Inorganic N content (mg N g_1) in the surface soil (0-5 cm) at the CP site (o) and at the SA01 site (•). (a) NH4-N, (b) NO3-N. Vertical bars represent + one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

(b) Days after burning

Figure 2: Inorganic N content (mg N g_1) in the surface soil (0-5 cm) at the CP site (o) and at the SA01 site (•). (a) NH4-N, (b) NO3-N. Vertical bars represent + one SE of the mean (n — 10). The SA01 site was burned on 16 August 2001. Superscript letters indicate the results of one-way ANOVA; means with the same letters are not significantly different by Scheffe's test at each site (p — 0.05).

(Fig. 3b-d). However, exchangeable cations in the topsoils had decreased within the 2 days following the first post-fire sampling. The concentrations of these exchangeable cations of the topsoils dropped during the period of the turnip crop, reaching levels similar to those at the CP site by mid-November.

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