Effects of long-term different substitution rates of organic fertilizers on soil nutrient dynamics in tea plantations

Objectives Substitution of partial chemical fertilizer nitrogen (N) with organic fertilizers is promoted for the sustainable development of tea plantation. We investigated the effects of different organic substitution rates (OSRs) on soil nutrient changes and tea yields for the optimization of fertilization strategy.

Methods  The research based on a positioning tea fertilization experiment, located in Ningde City, Fujian Province, started since 2017, in which the tea trees were planted in 2016. Eleven soil samplings were conducted within one production year (2018−2019) in the treatment plots of no nitrogen fertilizer control (CK), pure chemical fertilizer (NPK), and the plots of substitution of 25%, 50%, 75% and 100% of chemical fertilizer with pig manure compost (denoted as OM25, OM50, OM75, and OM100). Soil pH, and nutrient contents were measured, and the precipitations, ground surface temperatures, and tea yields were investigated at the same time.

Results All the fertilization treatments significantly increased soil nutrient contents, however, the coefficient of variation (CV) of soil nutrient contents increased with the enhanced organic substitution rate. Except for NH₄⁺-N, OM25 and NPK treatment were recorded similar soil nutrient contents and CVs, which were lower than in the other treatments. The average NH₄⁺-N content in OM25 treatment was 1.42−3.35 times higher than that in the other treatments (P<0.05). The annual CVs of NO₃⁻-N content exceeded 150% under the OM50−OM100 treatments, and no significant differences existed in the CVs among them. High OSRs increased the average contents of available Ca, Mg, Mn, Cu and Zn. The average available Ca, Mg, Mn, Cu and Zn contents in OM100 were 4.12, 13.83, 2.77, 12.32 and 36.32 times of those in CK and OM25. With the increase of OSRs, surface soil pH and readily available K content increased, while NH₄⁺-N content and fresh tea leaf yield decreased. Except for NH₄⁺-N, soil nutrient contents were negatively correlated with precipitations at 7 days before sampling, and soil mineral N content was negatively correlated with surface temperature at any stage.

Conclusions  Application of 100% organic fertilizer is not conducive to tea yield and soil nutrient supply due to the resulted high soil pH. Excessive rainfall and high surface temperature also exaggerate the unbalanced supply of soil nutrients. Low organic substitution rate could guarantee ampler and stable mineral nitrogen supply, and would not change soil pH in tea plantations, so is suitable for the growth of tea trees.