The six-year biochar retention interacted with fertilizer addition alters the soil organic nitrogen supply capacity in bulk and rhizosphere soil.

Nitrogen fractions in soil, like organic nitrogen, mineral nitrogen, and free amino acids, are sensitive pointers to the soil nitrogen pools involved in nutrient cycling. As a potential improvement measure, biochar might improve soil fertility and nutrient availability. However, few studies have focused on the long-term effects of biochar retention on the soil nitrogen supply capacity of bulk and rhizosphere soil in brown earth. Therefore, a six-year field experiment was conducted in 2013, concentrating on the impact of biochar retention on soil nitrogen fractions. Four biochar rates were tested: no biochar amendment (CK); 15.75 t ha-1 of biochar (BC1); 31.5 t ha-1 of biochar (BC2); 47.25 t ha-1 of biochar (BC3). Our results showed that the elevated application rates significantly enhanced soil organic matter (SOM), and total nitrogen (TN), and improved pH in both bulk and rhizosphere soils. Acid-hydrolyzable nitrogen (AHN) content in biochar treatments was higher than that of CK in bulk and rhizosphere soil. The content of non-hydrolyzable nitrogen (NHN) was increased in 47.25 t ha-1 of biochar retention. Ammonium nitrogen (AN) and amino sugar nitrogen (ASN) contents were higher in bulk soil than in rhizosphere soil. Neutral amino acid contents were the highest both in bulk and rhizosphere soil. Principal component analysis (PCA) showed that soil organic nitrogen was significantly influenced by BC3 treatment in bulk soil, and largely influenced by other treatments in rhizosphere soil. Partial least square path modeling (PLSPM) revealed that NH4+-N was mainly derived from amino acid nitrogen (AAN) and AN in bulk soil and AAN and ASN in rhizosphere soil. These results indicate that different biochar retention rates contributed to improve soil nutrients. Amino acid nitrogen was the prominent nitrogen source of NH4+-N in bulk and rhizosphere soils.

Spatio-temporal variabilities of soil phosphorus pool and phosphorus uptake with maize stover biochar amendment for 5 years of maize.

Phosphorus reuse by application of biochar is a recent concept that needs to be supported by long-term field data. To monitor biochar's long-term effects on P turnover, one-off biochar was applied in 2013 with mineral NPK fertilizers being applied every year since then. Biochar application rates included 0 t ha-1 (CK), 15.75 t ha-1 (BC1), 31.5 t ha-1 (BC2), and 47.25 t ha-1 (BC3). Over the 5 years' field experiment, P distribution in soil profile, inorganic and organic P fractions in bulk, and rhizosphere soil and maize P uptake were determined. The results showed that biochar reduced the inorganic P fractions (Ca2-P, Ca8-P, Al-P, Fe-P and O-P by 4.8-33.7%, 8.8-59.0%, 13.7-28.6%, 8.4-17.6%, and 3.3-25.5%, respectively), and increased organic P fractions (MLOP and HROP by 67.2-11.6% and 18.8-87.7%, respectively) in bulk soil, while in rhizosphere soil, Fe-P and MLOP were decreased by 13.4-34.5% and 67.2-111.6%, respectively, in 2017. After the application of biochar for 5 years, moderately labile organic phosphorus (MLOP), moderately resistant organic phosphorus (MROP), and highly resistant organic phosphorus (HROP) with different biochar treatments were enhanced by 12.8-42.7%, 20.1-48.0%, and 5.5-66.6%, respectively, but Ca8-P, Al-P, O-P, and Ca10-P were all decreased by 18.6-24.9%, 16.4-21.4%, and 3.3-23.48%, respectively. Total P storage in 0-100 cm was declined by biochar. Increases in maize P uptake in the stover (38.6-71.3%) and grain (20.9-25.5%) were occurred after 31.5 t ha-1 and 47.25 t ha-1 biochar addition. To sum up, biochar is found to regulate the distribution, storage, and transformation of soil P, which lead to increase in maize P uptake.