PE mulch has been used in agriculture since the 1950s due to the benefits it provides for weed suppression, soil temperature and moisture modification, and yield increase (Kasirajan and Ngouajio, 2012). In 2016, global use of PE mulch was 3.6 million tonnes and ≈106,000 tonnes in North America, and anticipated annual increase is 3.9% to 4.5% between 2017–2022 (MarketandMarkets, 2017). It was estimated in 2012 that 20 million ha of agricultural land was covered with PE mulch in China, and recycling has been limited because most of the mulch is thin (<8 μm) and difficult to retrieve (Liu et al., 2014). Elsewhere in the world, the contamination of mulch with soil and plant debris (50% or more by weight) prevents it from being recycled (Kasirajan and Ngouajio, 2012). Thus, the recycling rate of agricultural PE mulch is low, less than 10% in the United States and elsewhere (Briassoulis, 2006; G. Jones, personal communication), and the majority of agricultural mulch is landfilled, stockpiled, or even burned (Goldberger et al., 2015; Kasirajan and Ngouajio, 2012).
The use of BDM is increasing among vegetable growers in recent years because it can provide comparable benefits as PE mulch and can be disposed in place by tillage into the field (Cowan et al., 2014; Kasirajan and Ngouajio, 2012; Miles et al., 2012). It is important to note, however, that at this time in the United States, BDMs are advertised as biodegradable when tilled into the soil, but BDMs are not required to be tested for soil-biodegradability. In 2018, the European Union released a new standard to test plastic biodegradability in soil [European Norms (EN) 17033, 2018], and this standard is summarized by Hayes and Flury (2018). Most field studies use the term BDM to refer to products that are advertised as biodegradable, but most of these studies test functionality of the BDM during the cropping season and do not test biodegradation of the mulch either during the season or after soil incorporation.
Northwest Washington has a warm Mediterranean climate (Csb) (Peel et al., 2007), with average daily temperature of 15 °C June through August [Washington State University (WSU) AgWeatherNet, 2019], which influences soil temperature and relative humidity (RH). Black plastic BDM functions similarly to PE mulch as they both increase soil temperature, suppress weeds, and enhance yield (DeVetter et al., 2017; Ghimire et al., 2018; Zhang et al., 2019). Clear plastic mulch can be especially beneficial for production of warm season crops in regions with a Mediterranean climate, due to the increased soil temperature early in the season during a critical period for plant establishment and growth (Waterer, 2000). For example, Dabirian et al. (2017) reported that soil temperature under clear PE mulch was 5 °C higher than under black PE mulch from early June to mid-August in a watermelon (Citrullus lanatus) field in northwest Washington. Al-Assir et al. (1992) also reported that soil temperature under clear PE mulch was 1 to 2 °C higher than under black PE mulch in an unheated greenhouse located in Beirut, Lebanon.
Although clear plastic mulch can be beneficial in a region with a warm growing season, weed management can be a problem under clear plastic mulch as light transmission that warms the soil also allows weed development (Lamont, 2005; Waterer, 2000). Past studies have reported that clear plastic BDM tends to split during the growing season due in part to the heavy weed growth under the mulch, causing weed management concerns (Ghimire et al., 2020; Waterer, 2010). Thus, gains in crop development early in the season may be offset by competition due to increased weed growth under the mulch and when the mulch splits. There are few published studies exploring the use of clear plastic BDM in a Mediterranean climate, and it would be valuable to assess the potential relationships of soil temperature, weed growth, BDM splitting, and crop yield.
In contrast to plastic mulch, brown paper mulch reduced soil temperature by 1 °C in the open field and 1.6 °C in the high tunnel compared with black PE mulch in tomato production (Solanum lycopersicum) in a Mediterranean climate, but weed control and yield were similar to plants grown with black plastic BDM and PE mulch (Cowan et al., 2014). Ghimire et al. (2018) found that yield of pumpkin (Cucurbita pepo) grown with brown paper mulch was lower than PE mulch and several black plastic BDMs, but similar to one black plastic BDM and higher than bare ground in a Mediterranean climate. Miles et al. (2012) and Ghimire et al. (2018) also found reduced soil temperature under a brown paper mulch in Knoxville, TN, where the climate is subtropical. Similarly, Anderson et al. (1996) showed that a black paper mulch provided sufficient weed control and similar tomato yield as black PE mulch in Maine that has a humid continental climate. These different results could be due to mulch color, optimal soil temperature for crop growth, or other climatic conditions. Thus, it is worth studying whether a black paper BDM can provide similar yield as PE mulch in a Mediterranean climate.
BDMs are designed to degrade when exposed to biotic and abiotic conditions in the field (Brodhagen et al., 2015), and deterioration can occur during the growing season, especially where heavy fruit are in contact with the mulch, in some cases resulting in pieces of the BDM adhering to the fruit. For example, Limpus et al. (2019) reported that plastic BDMs adhered to melon (Cucumis melo) and pepper (Capsicum annuum) fruit but was easy to remove. In contrast, Ghimire et al. (2018) found that black plastic BDM and brown paper BDM pieces adhered to pumpkin fruit grown on a silt loam soil with drip irrigation, and adhered mulch was not easy to remove when fruit were wiped with a cloth several hours after harvest. Similarly, it has been reported that plastic BDM adhered to watermelon fruit and was difficult to remove after harvest in the Columbia Basin region of Washington State, which has a continental climate with sandy soil and low humidity conditions and where drip irrigation is used for crop production (R. Boydston, personal communication, 2009). It is important to note that PE mulch does not adhere to fruit.
The overarching goal of this study was to develop knowledge and practices regarding the use of black paper and plastic BDMs and clear plastic BDM compared with PE mulches for pumpkin production in a Mediterranean climate. The effect of time of day of harvest as well as time after harvest on removal of adhered mulch were also assessed. The specific objectives were to assess different PE and BDMs for 1) weed incidence, 2) soil temperature and moisture, 3) above-soil deterioration, 4) pumpkin yield, and 5) mulch adhesion on pumpkin fruit.
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