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To address the low productivity of sandy farmlands, our study aimed to conduct a comparative study on the effects of different organic amendment (OA) inputs for the potential improvement of crop yield and soil quality in sandy alkaline farmlands through the selection of a suitable OA. This study set up straw (ST) returning as control and chemical fertilizer (CF) treatment as a side control, and chose three OAs returning as treatments, including pig manure (PM), biogas residue (BR), and straw biochar (BC), for improving soil fertility, with all amendments having matched doses of nitrogen (N). The experiment was conducted at the Wuqiao Experimental Station (37°41 N, 116°37 E) of China Agricultural University in Hebei Province, China, from October 2012 to September 2016. The cropping rotation was the winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) rotation system. Through a consecutive four-year field experiment, the principal results showed that three types of OA application significantly increased soil organic carbon from 1.46 g kg−1 to 8.24 g kg−1, soil total N from 0.21 g kg−1 to 0.64 g kg−1, soil available potassium from 55.85 mg kg−1 to 288.76 mg kg−1, and soil available phosphate from 4.86 mg kg−1 to 65.00 mg kg−1 in the 0–20 cm soil layer. The BR was the most effective in improving soil nutrients as compared with the ST. The PM and BR treatments were more conducive to promoting crop yield by 6–20% than ST, and the BC treatment significantly reduced the yield of winter wheat by 19% and summer maize by 8%. As the BR and PM treatments improved the soil nutrient content and significantly increased crop yield, these were the top choices for transforming the low-yield sandy farmlands.

Soil aggregates provide microhabitats for microorganisms. However, the bacterial communities within various aggregate size fractions under organic amendments have rarely been evaluated especially in sandy soils. A field experiment involving addition of biogas residue (BR), pig manure (PM), biochar (BC), and straw (ST) of a sandy soil was performed to determine the structure of the bacterial community in aggregate size fractions by using high‐throughput sequencing. We found that organic amendments change soil aggregate fractions and nutrient content, alter bacterial diversity and composition, and contribute to the predicted bacterial functions. The BR increased bacterial α‐diversity in <0.25‐mm aggregates. The Nitrospirae was abundant in almost all aggregate sizes, whereas the abundance of Actinobacteria decreased in most of aggregates following BR addition. The PM amendment primarily facilitated the growth of Bacteroidetes and inhibited the growth of Acidobacteria and Chloroflexi in 0.25‐ to 2‐mm aggregates, whereas bacterial α‐diversity was decreased in the >0.25‐mm aggregates. The BC treatment decreased the bacterial α‐diversity in the <0.25‐mm aggregates and the Gemmatimonadetes abundance in all <2‐mm aggregates, but the Actinobacteria abundance was significantly increased in the >0.053‐mm aggregates. Collectively, the effects of organic amendments on the bacterial community varied greatly depending on type of amendments and aggregate size. The BR supported highly diverse bacterial taxa in the <0.25‐mm aggregates, whereas it proliferated N cycling‐related bacteria in all aggregates. Our results highlight the links between bacterial community and aggregate size fractions in sandy soils depending on organic amendments.

Diversified cropping systems can enhance soil condition and increase system productivity worldwide. To reduce the negative effects that accompany the continuous winter wheat–summer maize (WM) double-cropping in the North China Plain (NCP), diversified crop rotation (DCR) needs to be considered. The objective of this study is to evaluate the effect of DCR on soil health and wheat productivity as compared to a continuous WM double-cropping. A field experiment (37°41′ N, 116°37′ E) was established in the NCP including a traditional WM double-cropping as a baseline. During 2016/2017–2017/2018, the control is winter wheat–summer maize→winter wheat–summer maize (WM→WM) and seven DCRs as follow: fallow→winter wheat–summer maize (F→WM); spring maize→winter wheat–summer maize (Ms→WM); winter wheat→winter wheat–summer maize (W→WM); sweet potato→winter wheat–summer maize (Psw→WM); spring peanut→winter wheat–summer maize (Pns→WM); winter wheat–summer peanut→winter wheat–summer maize (WPn→WM) and potato–silage maize→winter wheat–summer maize (PMl→WM). Our results indicated that DCRs significantly changed certain soil health indicators in 2016/2017 compared with the control, where F→WM rotation significantly decreased soil pH by 2.7%. The DCRs, especial Psw→WM and Pns→WM rotations showed a potential positive effect on soil health indicators at the end of the second year (2017/2018) compared with the control, where sweet potato increased soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), urease activity (UA) and alkaline phosphatase activity (APA) in 2017/2018 by 5.1%, 5.3%, 13.8%, 9.4%, and 13.5%, respectively. With the spring peanut, TN, AP, and soil APA were increased by 2.1%, 13.2%, and 7.7%, respectively. Although fertilizer and irrigation input of DCRs were lower than the control, no significant decrease was observed on actual wheat yield as compared to the control (7.79 Mg/ha). The finding of this study highlights the value of DCRs, especially, Psw→WM and Pns→WM rotations over WM double-cropping in the NCP.

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