Aged biochar nutrient release trends were similar to that of fresh biochars in regard to greater nutrient release from grass versus oak and from low versus high temperature biochars (Fig. 1d, e, f). Most nutrient concentrations in leachate from aged biochars similarly stabilized by the third water addition (after 120 mL), but there was a greater degree of variability compared to fresh biochars. On a cumulative basis, aged biochars released only 5–37% P than fresh biochars. Unexpectedly, however, both oak and grass high temperature biochars released about a third more DOC than fresh biochar. Also, aged and fresh grass biochars released similar cumulative amounts of TKN, but aged oak biochars released two to six times greater amounts of TKN than freshly made biochars.
Nitrogen and phosphorus speciation in the initial and final batch leachates (i.e. first and fifth 50 ml DI water extraction) collected are shown in Table 1 and percent organic compositions are calculated in Supplemental Table S3. Nitrate concentrations in both the initial and final biochar leachates (0.08 mg L− 1, on average, or 0.16 mg g− 1 biochar) were low. However, while nitrate represented only between 2 and 14% of the total N leached from most of the chars, it represented 36 to 55% of the N lost by those chars which leached very little N (Oak-650 and Oak-250 in later stages of batch leaching). Organic N was absent from oak biochar leachates, but represented about 60% of the N in the initial grass biochar leachates and closer to 80% in the final ones (Supplemental Table S3). Organic P represented between 39 and 83% of the total P in biochar leachates initially, but was absent in the final biochar leachate.
Aged biochar nutrient release trends were similar to that of fresh biochars in regard to greater nutrient release from grass versus oak and from low versus high temperature biochars (Fig. 1d, e, f). Most nutrient concentrations in leachate from aged biochars similarly stabilized by the third water addition (after 120 mL), but there was a greater degree of variability compared to fresh biochars. On a cumulative basis, aged biochars released only 5–37% P than fresh biochars. Unexpectedly, however, both oak and grass high temperature biochars released about a third more DOC than fresh biochar. Also, aged and fresh grass biochars released similar cumulative amounts of TKN, but aged oak biochars released two to six times greater amounts of TKN than freshly made biochars.
Nitrogen and phosphorus speciation in the initial and final batch leachates (i.e. first and fifth 50 ml DI water extraction) collected are shown in Table 1 and percent organic compositions are calculated in Supplemental Table S3. Nitrate concentrations in both the initial and final biochar leachates (0.08 mg L− 1, on average, or 0.16 mg g− 1 biochar) were low. However, while nitrate represented only between 2 and 14% of the total N leached from most of the chars, it represented 36 to 55% of the N lost by those chars which leached very little N (Oak-650 and Oak-250 in later stages of batch leaching). Organic N was absent from oak biochar leachates, but represented about 60% of the N in the initial grass biochar leachates and closer to 80% in the final ones (Supplemental Table S3). Organic P represented between 39 and 83% of the total P in biochar leachates initially, but was absent in the final biochar leachate.
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