Environmental changes in Yellow River Delta with terrace construction and agricultural cropping

Sampling and measuring

In September 2017 and 2018, we conducted field surveys near a typical rural village (Maotou village, 118.49°E, 37.82°N) in the Yellow River delta (Fig. 1). It is approximately 80 km away from the new mouth of the Yellow River. It is in the earliest formed part of the delta and is representative for terrace construction in this area. The annual average temperature is approximately 12 °C; the annual precipitation is approximately 580 mm and mainly occurs from June to August. The lands were made of sediments that were deposited by the old Yellow River between 1855 and 1976 (Yang, 2012). In the 1990s, locals started constructing terraces in this area. Currently, these terraces have been used for growing crops such as cotton and maize. These croplands are irrigated only once a year in spring for sowing, and the fresh water is drawn from the Yellow River rather than using underground water. A few areas of crude flats are still reserved for folk sacrifice ceremonies.

We selected three land types within the terrace landscape (Fig. 1): crude flats that remained in natural state; terrace slopes (30–50°), which skirt the terraces; and terrace croplands, which are leveled tops of the terraces and have been subject to tillage, cropping and fertilization for approximate 20 years. The weed survey was done in September 2017 when weeds grew quite well, and the soil survey was completed in 2018. For each land type, we surveyed weeds in 40 random quadrats (1 × 1 m), measuring the cover and height of each weed species. Then, we selected four random points from each land type for soil sampling. At each of these points, we sampled soil at three layers: 0–10, 10–20, and 20–30 cm deep. Moreover, we sampled a soil cube (10 × 10 × 10 cm) from each of these layers. These samples were taken to laboratory for further analysis. The cube samples were washed so that live roots and rhizomes, namely, underground organs (UO), remained. These UOs were oven-dried at 85 °C for 2 days and weighed using an electronic scale (precision = 0.01 g).

Other soil samples were air-dried, fully ground, and sieved with a 20-mesh wire screen. We measured pH, salinity, organic matter, total N, and total P of the sieved soil samples. We took 10 g from each sample and mixed the subsamples with pure water at the mass ratio of 1:5 (soil:water). We measured pH of steady clear solution of the subsamples using an electronic meter (ST2100/F, OHAUS in Shanghai), and measured salinity of the solution after fully drying and H₂O₂ solving. Organic matter (OM) in the soil was measured using the potassium bichromate titrimetric method; total N (TN) was measured using the Micro-Kjeldahl method; total P (TP) was measured using the Mo-Sb colorimetric method (Bao, 2008).

Data analysis

We examined variations in the soil salinity, pH, OM, TN, TP, and dry UO among the three land types and soil layers using two-way ANOVA (James et al., 2013). This method requires the examined data to follow a normal distribution. As a supplement, we also conducted another analysis using the nonparametric Kruskal-Wallis test, which does not require normally distributed data (Hollander & Wolfe, 1973). The Kruskal-Wallis test is similar to the method of one-way ANOVA, while takes advantage of the relative ranks of the data. All these analyses were completed using the software R4.0.3 (R Core Team, 2020).

For evaluating the weeds, we used a composite index, the product of coverage and height (PCH) of weed species in a quadrat, thus quantifying the standard cover thickness of a weed species. Then, we classified all weeds by their life forms as follows: single (grows as a standing singleton), clump (grows as a clump of numerous tillers), vine (grows as an aboveground vine) and rhizome (grows underground with rhizomes to generate new ramets). In addition, we classified these weeds by their life span (annual vs. perennial). Finally, we summed the PCHs of each weed type in a quadrat and averaged the PCH sums of a certain weed type among the 40 quadrats of the same land type.

Positive changes with the terracing and cropping

Soil salinity in the terrace croplands was significantly lower than in the crude flats. After terraces were built using the deposited soils, rainfall must have accelerated the desalination process. Some years later, salinity was no longer a hindrance to growing certain varieties of cotton and maize; thus, the locals began growing them on the terraces. Harvesting the crops could also contribute to salinity reduction in the lands (Zhao et al., 2013). Presently, soil salinity in the terrace croplands has reduced to <4 g·kg⁻¹, whereas in the crude flats, remains >6 g·kg⁻¹.

Weeds on the terrace slopes grew considerably thicker than in the flats and croplands, and the richness of weed species was also the highest on the terrace slopes. This is partly attributed to farmers’ intention on permitting weed growth because weeds (particularly perennials) can protect the terraces from potential eroding and collapsing for storms (Liu et al., 2013; Asimeh et al., 2020). In the croplands, weeds were scarce because of farmers’ tillage and weeding activities; in the crude flats, they grew only slightly better than in the croplands perhaps owing to high soil salinity and compaction (Bennett, Barrett-Lennard & Colmer, 2009; Xia et al., 2009). This indicates that terrace slopes can be refuges for conserving a considerable number of plant species, which can protect the croplands against rainfall erosion.

Negative changes with the farming

Unfortunately, soil pH in the terrace croplands was higher (>9) than in the crude flats (<9). Soil OM content, particularly at depths >10 cm, was lower in the croplands than in the flats. These changes must reduce soil fertility in the croplands (Chapagain & Raizada, 2017; Zhang et al., 2020). They are considered to be a chronic response to the current farming regime, rather than to the terracing practice, because these changes were not as evident in the terrace slopes.

Because of agricultural practices in the terrace croplands, fertilizers and herbicides were repeatedly added to eradicate weeds and to replenish N and P contents in the soils. However, these chemicals could cause cumulative changes in the soils (Zalidis et al., 2002; Kaur, Kaur & Chauhan, 2018). Some of the commonly used chemical fertilizers, such as diammonium phosphate and urea, can slightly increase soil pH (Zhou et al., 2004; Jin et al., 2008). The increase may be small initially; however, it could accumulate year after year, and now has turned so significant. Aboveground and underground amounts of weeds (plants) were observed to be extremely low in the croplands, thus the supply of soil organic matter from them must be not enough to compensate for the yearly decomposition of soil OM that must have been hastened by the tillage (Hobley et al., 2018; Storkey & Neve, 2018).

Obscure changes of soil total N and P

The total N and P contents in the soils seemed to be equivalent among the three land types. As the most-required elements by crops, soil N and P must be repeatedly extracted from the soils with annual crop harvest. To compensate for the loss of N and P, locals added chemical fertilizers, such as diammonium phosphate and urea. The N and P contents in the soil seemed to be maintained and were not significantly lower than in the crude flats and terrace slopes. However, this is only a judgment based on averages.

The N and P contents in the soil were dependent on soil depths. For soil layers at the depth of 20–30 cm, N and P were observed to be much lower in the croplands than in other lands. This indicates that the added chemical fertilizers could not supplement the N and P loss in the subsoil >20 cm deep. If the loss cannot be timely reversed, soil degradation will become more and more severe, and ultimately make the terrace lands unsuitable for agriculture production (Dai et al., 2017; Jensen et al., 2020). As thus, we think that it is time to convert the current land use regime into a new one that should well restore fertility of the terrace soils (Ordóñez-Fernández et al., 2018; Fu et al., 2020). For example, locals can be encouraged to grow legume crops or forage such as alfalfa whose roots can feed deep soil with multifunctional biotic fertilizers, avoiding excessively relying on chemical fertilizers (Hu et al., 2019). This suggestion is also informative for other coastal areas annoyed with thus saline lands.