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

Objectives In the context of the sustainable development policy, the substitution of synthetic nitrogen (N) for organic fertilizer has been promoted in tea plantations. Investigating the effects of different organic substitution rates on soil nutrient dynamics and tea yield will have an important reference value for optimizing organic substitution rates (OSRs) in tea plantations.

Methods In the long-term field experiment, soil samples were collected from six treatments for 11 times within one production year (2018-2019), including the control treatment (CK, without N addition), pure synthetic fertilization (NPK), 25% OSR (OM25), 50% OSR (OM50), 75% OSR (OM75), and 100% OSR (OM100). Soil nutrients and tea yield were measured, and precipitations and ground surface temperatures were collected as well to analyze the effects of OSRs on soil nutrient dynamics and tea yield.

Results Fertilization significantly increased the nutrient content in soil, but the coefficient of variation (CV) of soil nutrient contents increased with the OSR. Except for NH₄⁺-N content, there was no significant difference in the nutrient content and CV between NPK and OM25% treatment, both of which were lower than other treatments. The average content of NH₄⁺-N in OM25 treatment increased by 1.42−3.35 times compared with other treatments, which was significantly higher than that under other treatments (P<0.05). The CV of NO₃⁻-N content exceeded 150% under OSR treatments, and its content gradually increased with the increase of OSR but showed no significant difference (P>0.05). The higher OSR treatments increased the average contents of Ca, Mg, Mn, Cu and Zn, and their average contents were respectively increased by 4.12, 13.83, 2.77, 12.32 and 36.32 times under OM100 treatment when compared with the lowest treatment. With the increase of OSR, surface soil pH and available potassium content increased, but NH₄⁺-N content and fresh leaf yield decreased. Except for NH₄⁺-N, there was a significant negative correlation between soil nutrient contents and precipitations 7 days before sampling, and there was a significant negative correlation between soil mineral N content and surface temperature at any stage.

Conclusions 100% organic fertilization input is not conducive to an increase in tea yield and soil nutrient supply, and increasing the soil pH has a significant limiting effect on the growth of tea plants. Excessive rainfall and high surface temperature will also aggravate the risk of nutrient loss. Overall, lower OSRs could stabilize the soil nutrient content in tea plantations in our study. Meanwhile, adjusting the topdressing time by combining a 7-day weather forecast could reduce the risk of nutrient loss to a certain extent.