1. INTRODUCTION
Globally, approximately 1.1 ×
109 ha of lands are affected by soil salinity (Hopmans
et al., 2021), which is threatening agricultural production, soil
health, and food security (Sahab et al., 2020). China, for instance, has
approximately 3.6 × 107 ha of saline soils,
which possess great potential for
agricultural utilization after amelioration (Rezapour et al., 2017; Yang
et al., 2022). Saline soil plays a vital part in global carbon (C) and
nitrogen (N) cycling, especially under the conditions of human
intervention (Setia et al., 2013). Additionally, saline soil has low
soil C and N contents, which intensify the negative effects of soil
salinization (Huo et al., 2017).
Straw returning is widely recommended to maintain soil C and N contents
through efficient use of the crop straw resource (Xia et al., 2018).
Straw deep returning as an interlayer is an effective farming strategy
for saline soil amelioration (Zhao et al., 2013a), which mitigates soil
salinity (Zhao et al., 2013b), improves soil structure (Cong et al.,
2019), regulates soil microbial community (Li et al., 2016), and boosts
crop yield (Zhao et al., 2016). Soil C and N in farmland was mainly
derived from crop straw, which affects soil C and N availability after
mineralization (Lian et al., 2015). However, contradictory results were
observed with different amounts of straw returning (Jin et al., 2020),
due to local climatic conditions and soil properties (Hao et al., 2022).
Excessive straw returning usually resulted in an undesirable soil C:N
ratio (Zhang et al., 2015), prevented straw mineralization (Shahbaz et
al., 2017a), and reduced crop production (Islam et al., 2022). However,
insufficient straw amendment weakened soil C and N sequestration (Zhang
et al., 2016), and did not meet the balance of soil C and N pools
(Andruschkewitsch et al., 2013). Thus, it is critical to quantify the
impacts of various amounts of straw deep returning as interlayers on
soil C and N, especially in saline soils.
Inability to incorporate straw into deep soil constrain the C and N
sequestrations (Zhao et al., 2018), while the legacy effects of straw
deep returning on soil C and N remain unclear in saline soils (Zhai et
al., 2021). The SOC content in 20-40 cm soil depth was increased in the
first two years, while decreased from the third year under the condition
of straw deep returning with 12 Mg ha-1 (Cong et al.,
2019). However, this trend was influenced by the amount of straw input
(Zhang et al., 2021), and it needed further investigation for saline
soil (He et al., 2022). Meanwhile, soil C and N to straw deep returning
varied widely at different soil depths (Dikgwatlhe et al., 2014). Straw
deep returning (20-40 cm) increased SOC content by 8% in topsoil (0-20
cm), and up to 27% in the subsoil (20-40 cm) in the third year (Zheng
et al., 2021). These resulted in different
soil stratification ratios, a
soil quality indicator of nutrient distribution in soil profile (Melero
et al., 2012). Similar or opposite responses of soil TN content to straw
returning were detected, compared to SOC content (Pittelkow et al.,
2015), which led to varied soil C:N ratio. Thus, it is necessary to
determine the impacts of straw deep returning as interlayer on SOC, TN,
C:N ratio, and stratification ratio, considering the amount of straw
input (Cong et al., 2019), experimental year (Berhane et al., 2020), and
soil depth (Liu et al., 2021).
Therefore, we conducted a four-year field experiment with four levels of
straw interlayers in the Hetao Irrigation District. We hypothesized that
1) the SOC increased in the first several years after straw deep
returning, while decreased from
subsequent years, it ranged according to the amount of straw input (Cong
et al., 2019) and soil depth (Latifmanesh et al., 2020); and 2) straw
deep returning led to similar trend between TN and SOC, which resulted
in similar soil C:N ratio, while remarkable lower soil stratification
ratios (Liu et al., 2021). Thus,
we aimed to 1) quantify the effects of straw interlayers with different
input rates on SOC and TN within 0-80 cm soil depth in four years after
straw deep returning; 2) explore the relationships between straw
interlayer and C:N ratio / stratification ratios, aiming to higher soil
quality and productivity in a semi-arid saline soil.