Aging of biochar in a Cambisol for 13 years under organic farming field conditions affects its chemical structure but still shows positive impacts on plant growth

Abstract

Soil deterioration due to excessive fertilization and climate change has adverse effects on crop growth globally. In order to enhance soil fertility, application of organic soil amendments has gained importance. One of them represents biochar which is recognized as a sustainable agricultural practice with significant benefits for soil health as it bolsters fertility, enhances nutrient retention and soil structure while sequestering carbon on a long-time scale. However, during aging biochar has been shown to change its properties. Knowledge about those alterations and changes in performance as soil amendment is still scarce. Therefore, our research aimed to compare the impact of freshly added biochar to that of biochar and co-composted biochar having been allowed to age under natural organic farming field conditions for 13 years in a Cambisol on soil properties and the development of Lactuca sativa L. var. The plants were grown in pot experiments for 9 weeks under controlled greenhouse conditions. Compared to the soil freshly amended with biochar (5.93%), the organic matter content of the soils with the aged biochar and aged co-composted biochar was lower (4.76% and 4.91%, respectively) suggesting that some of the biochar was lost during 13 years of aging. However, their SOM content was still significantly higher than in the untreated control soil (3.62%), indicating a positive long-term effect of biochar treatment on soil carbon sequestration. Solid-state 13C NMR data showed significantly higher aromaticity of the soil amended with fresh biochar compared to the control soil, which can be directly linked to the polyaromatic nature of biochar. As indicated by the 13C NMR spectra, aging of biochar resulted in a relative loss of aromatic structures and a relative increase of O-alkyl C and alkyl C confirming the oxidation of biochar with ongoing aging. Addition of fresh biochar decreased the relative amount of available nitrogen forms (nitrates and ammonia) although its contribution to the soil increased with aging. Even after aging, biochar treatments resulted in a larger production of fresh and dry biomass if compared to the control soil. We noted that co-composted biochar led to greater photosynthesis indexes (SPAD and QY), better water use efficiency (WUE), nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE). In addition, it increased the nutrients contents of the plants. This study also explored the multiple positive and negative interactions between different types of biochar addition as organic soil amendments and plant physiological traits. For this purpose, statistical analysis was performed as analyses of variance (ANOVA). Principal Component and correlation analysis (PCA) were also tested. The results of this study will help in understanding the complex relationships between soils, amendments, and plants, and as such are vital for optimizing soil health and achieving higher crop yields through organic amendments.   Acknowledgements: Funded by the European Union. Grant agreement No. 101059546, María Rocio Reinoso and Marta Velasco-Molina are thanked for their technical help in the laboratory.