¹⁵N labelling of cattle manure reveals the distribution of organic fertiliser nitrogen in a winter wheat system

Highlights

• ¹⁵N tracer technology was used to study N supply from livestock manure to crop plants and the soil.

• On the basis of yield data, the optimal treatment was 30 t ha^–1 cattle manure with 225 kg ha^–1 N fertiliser.

• A low rate of N fertiliser (75, 150 kg ha^–1) with manure (M+N75, M+N150) increased plant uptake of manure N.

• Up to 8.1% of crop N uptake in the M+N150 treatment was derived directly from manure.

• Approximately 37.2–53.1% of the applied manure N was recovered in the total N pool at 0–20 cm soil depth.

Abstract

¹⁵N tracer technology can be used to study the distribution of fertiliser N in the soil–plant system. However, few studies have used ¹⁵N tracer to directly label organic fertiliser such as livestock manure. Therefore, we analysed the distribution of N derived from cattle manure labelled with¹⁵N in winter wheat plants and the soil. An unlabelled large-plot experiment and ¹⁵N-labelled micro-plot experiment were performed using five fertilisation treatments: M (30 t ha^–1 manure) without additional N fertiliser, M+N75 (75 kg N ha^–1), M+N150 (150 kg N ha^–1), M+N225 (225 kg N ha^–1) and M+N300 (300 kg N ha^–1). The amounts of manure-derived N in crop plants and the soil were determined over a 2-year period (2019 and 2020). The wheat yield of M+N150 treatment plots was close to the highest wheat yield obtained in the M+N225 treatment plots (6311 kg ha^–1 in 2019 and 6523 kg ha^–1 in 2020). A large amount of manure-derived N was distributed in the whole plants treated with M+N150, being 78.4% (2019) and 58.3% (2020) higher than that in the plants treated with M+N300. The N use efficiency of manure in the M+N150 treatment plots was 8.1%, not unlike the highest N use efficiency observed in the M+N75 treatment plots (8.4%, sum of 2019 and 2020). The manure-derived total N at 0–40 cm soil depth in the M+N150 treatment plots was lower than that in the M treatment plots, whereas the manure-derived mineral N in the M+N150 treatment plots was similar to that in the M treatment plots and lower than that in the M+N300 treatment plots. These results suggested that the application of 30 t ha^–1 of cattle manure with 150 kg ha^–1 of N fertiliser was the optimal fertilisation treatment in the soil–wheat system, which could obtain high crop yield and improve N use efficiency by increasing the distribution of manure-derived N in whole plants and reducing manure-derived N in the soil.

Introduction

Livestock manure contains rich nutrients such as nitrogen (N), phosphorus (P) and potassium (K) (Rajan and Anandhan, 2015). Consequently, manure application can stimulate plant growth and increase crop yields by improving soil fertility (Gai et al., 2018, Naeem et al., 2017, Omara et al., 2017) and enhancing soil microbial and enzymatic activities (Lupwayi et al., 2005, Chen et al., 2017, Kotroczó et al., 2014). In China, a large amount of livestock manure is produced every year; in 2019, the amount of organic N in livestock manure was 16.77 million tonnes, whereas the amount of inorganic N in agricultural fertiliser was 26.87 million tonnes (FAOSTAT, 2021). Currently, livestock manure is a misplaced resource that is not fully utilised.

Excessive inputs of inorganic N fertiliser cause problems such as high cost, nitrate pollution and loss of soil carbon (C) (Lassaletta et al., 2014, Schoebitz and Vidal, 2016). Reasonable application of organic and chemical fertilisers can reduce the singular dependence of crops on chemical fertiliser, minimise nutrient loss and improve nutrient use efficiency (Zhao et al., 2009, Wang et al., 2018). Evidence suggests that a combination of manure and chemical fertiliser is an efficient approach to ensure soil productivity and environmental quality (Nkoa, 2014). If livestock manure can be used judiciously in agriculture, it will reduce the amount of chemical N fertiliser applied and thereby prevent the problems associated with excessive chemical fertilisation.

The effects of different ratios of manure to chemical fertiliser on the soil–plant system have been widely investigated (Riley, 2015, Wang et al., 2017, Aula et al., 2019). For example, Guan et al. (2011) showed that the use of chicken manure to replace 50% inorganic N and reduce 20% inorganic N promoted 8.3% N nutrient absorption by rice and contributed to the maintenance of crop yield compared with 100% inorganic N. Another experimental study also indicated that cattle manure (267 kg N ha⁻¹) could be applied once over a four-year period to maintain winter wheat yield while improving soil pH and organic C content compared with the control (Aula et al., 2019). In addition, indirect methods based on laboratory incubation have been used to roughly measure the contribution of manure N to the soil. For example, Calderón et al. (2005) enclosed manure in mesh bags and buried it in the soil; after incubation, the changes of manure N content in mesh bags were measured to calculate the contribution of manure-derived N to soil N. In addition, Mohanty et al. (2011) calculated the contribution of manure-derived N to soil N by subtracting the mineral N content of soil treated with inorganic N alone from that of soil treated with combined manure and inorganic N after incubation. However, the fate of manure N in the soil–plant system and its relative contribution to the soil and crop plants have not yet been accurately calculated.

When compared with indirect measurement methods, ¹⁵N tracer technology allows for accurate and direct measurement of nutrient flow in manure and enables the measurement of manure-derived N distribution in the soil (Powell and Wu, 1999). Powell et al. (2004) reported that high levels of manure ¹⁵N enrichment can be used for long-term manure N cycling studies. Furthermore, Munoz et al. (2003) showed that the use of ¹⁵N-labelled manure supports the accurate measurement of total soil N and estimation of soil N balance. Therefore, we applied ¹⁵N-labelled cattle manure in combination with different rates of N fertiliser in a winter wheat system over a 2-year period. We then determined the amounts of manure-derived N in crop plants and the soil to accurately calculate the distribution of N derived from livestock manure and its relative contribution to crop plants and the soil. The results of this study could provide support for appropriate manure utilisation with respect to crop yield and N use efficiency.