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Resource landscape, microbial activity, and community composition under wintering crane activities in the Demilitarized Zone, South Korea

['Kyungjin Min', 'Center For Anthropocene Studies', 'Korea Advanced Institute Of Science', 'Technology', 'Daejeon', 'South Korea', 'Myung-Ae Choi']

Date: 2022-06

Endangered cranes migrate to the Demilitarized Zone in Korea in search for habitat and food during winter. While cranes have the potential to influence soil biogeochemical processes via dropping, foraging, and walking, few studies have investigated ecological roles of migrating birds in the new habitat. Here, we explored how cranes alter resource landscape (the amount and quality of carbon) and microbial community in soil. We set up control (fenced, no crane access) and treatment (free crane activities) plots (n = 6, respectively) in a rice paddy, and collected soils at 0–15 cm three months after the crane migration. Soils were tested for total carbon, total nitrogen, water extractable organic carbon, and Diffuse Reflectance Infrared Fourier Transform Spectroscopy, along with microbial parameters (biomass, respiration, community composition). The wintering crane activity significantly increased total carbon and nitrogen contents, but decreased the ratio of CH (aliphatic) to COO (carboxylic) in soil. Also, both microbial biomass and respiration was greater in soils under crane activities. Bacterial and fungal community composition differed with or without crane activities, with treatment soils harboring more diverse microbial communities. Our results demonstrate that crane migration created a distinct system with altered resource landscape and microbial community, highlighting beneficial effects of migratory cranes on the soil biogeochemical processes in rice paddies. This study may help encourage more farmers, local governments, and the public to participate in crane conservation campaigns targeted at rice fields.

Introduction

Habitat loss, excessive hunting, and poisoning have decreased the number of cranes, leading to 11 out of 15 crane species threatened with extinction under the International Union for Conservation of Nature (IUCN) Red List Categories system [1]. Once common wintering birds in the Korean Peninsula [2], cranes are now observed in few areas including the Demilitarized Zone (DMZ) and Civilian Control Zone (CCZ) in South Korea. Previous studies have documented the crane migratory pathways [3, 4], population change [5, 6], and foraging behavior [7]. In spite of these efforts, however, we do not know the effects of crane migration on the biogeochemical processes in the new habitat. Understanding the direction and magnitude of the changes in biogeochemical processes after the crane migration would help us better assess ecosystem functions during winter, a critical step to evaluate beneficial effects of cranes and to obtain a momentum for the crane conservation efforts among local farmers, governments, and the public.

Cranes may alter biogeochemical processes of the new habitat by recharging, redistributing, and refreshing resources during winter. Bird fecal matter is rich in carbon (C), nitrogen (N), phosphorus, and potassium [8–10], likely serving as fertilizer. As such, ecosystems with limited resource availability can be influenced by the input of bird fecal matter. For example, Firth et al. [10] demonstrated that winter migration of goose and duck into rice fields increased total C and N concentration in soil, with a potential to replace 8.2–27.5% of the fertilizer recommendation dose. Also, cranes may redistribute resources within and across ecosystems. Because cranes are omnivores feeding on fish, insects, snails, and grains [1], they may transfer nutrients across ecosystems by taking up relatively N-rich fish or aquatic invertebrates from streams or reservoirs and defecating in other places. Using 15δN, Kameda et al. [11] found that great cormorant (Phalacrocorax carbo) transports N from fresh water to forest ecosystems via droppings and collection of N-rich nest material. Alternatively, birds may refresh the quality of resources via digestion or foraging. Bird fecal matter is partly or completed digested, so it may provide readily available resources to other organisms [10]. Or, foraging itself can alter the amount, chemical composition, and physical condition of rice straw left in the field after harvest [12].

Changes in the resource quantity and quality often alter soil microbial activity and community composition, with no clear directional changes [13–16]. A meta-analysis showed that microbial biomass and respiration decreased with N additions in all ecosystems [17]. Likewise, N additions decreased a suite of microbial extracellular enzyme activity that breaks down soil organic matter [18, 19] and led to less diverse fungal communities [20]. Using modeling and meta-analysis approaches, Whittinghill et al. [21] demonstrated that decreases in the rate of microbial activity under N additions were due to increases in less bioavailable, slow-cycling C pool. Alternatively, increases in the resources can enhance soil organic matter decomposition. Soil microbial growth and respiration in rhizosphere are often higher than those in bulk soils, due to root exudates that contain readily-available resources to microbes [22, 23]. Microbial diversity was higher at surface soil than microbial diversity at 2 m of soil depth due to greater C availability [24]. High N availability led to microbial communities with more bacteriodetes and proteobacteria, and less acidobacteria [25]. Because soil microbial communities mediate biogeochemical processes [26, 27], any changes in their activity and community composition are likely to affect nutrient cycling in agroecosystems in the DMZ.

Our study site is Cheorwon, Kangwon Province, South Korea. Red-crowned cranes (Grus japonensis) and White-naped cranes (Grus vipio) and 5 other crane species come to Cheorwon in mid-October, and leave early March for north-eastern China and Russia for breeding [2]. Both Red-crowned cranes and White-naped cranes are endangered species, classified as Vulnerable at the IUCN’s Red List [28, 29]. Cranes were observed in Cheorwon in the 1970s for the first time since the Korean War (1950–1953), and their population started to pick up in the mid-1990s. The number of Red-crowned cranes grew from 372 in 1999 to 1,126 in 2021, while White-naped cranes showed a 10-fold increase from 474 to 5,330 during the same period [30]. Since the early 2000s, conservationists, environmental authorities, and local farmers started to take actions to protect cranes during winter. These practices include creating roosting places by flooding rice paddy fields, in addition to securing food sources by keeping rice straw in field after harvest, sprinkling out grains, and leaving the fields unplowed. By doing so, they hope to protect the endangered cranes, and to promote eco-tourism and the brand value of the rice they produce.

Our goal was to assess the effects of migratory wintering cranes on the biogeochemical processes in rice paddy agroecosystems around the DMZ [31]. We hypothesized that crane activities will increase the quantity and quality of resources in soil, and that microbial biomass, respiration, and diversity will be higher under crane activities. To test these, we collected soils at 0–15 cm three months after the crane migration from treatment (free crane activities) and control (fenced, no crane access) plots (n = 6, respectively). We assessed microbial activity (basal respiration), biomass (DNA content, substrate-induced respiration), and microbial community composition (fungi and bacteria), along with soil chemistry (total C, N, water extractable organic carbon (WEOC), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT)). To our knowledge, this is the first attempt to consider wintering cranes as biogeochemical modifiers in DMZ. By examining the impact of crane activities on agricultural soils, the results of this study can attract interests from researchers, farmers, and general public in conserving the endangered cranes.

[END]

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