1. Introduction
For future vertical farming systems, rabbits are considered the promising animal compared with poultry and fish. Plant food wastes can be fed to rabbits directly and rabbit manure can be used for plant production, thus realizing the cycle of planting and raising [
1]. At present, rabbits have become the second largest farmed species in Europe from the perspective of the breeding quantity. Similarly, China also has a large rabbit breeding volume of 313 million, corresponding to significant rabbit manure production of approximately 8 million tons [
2]. However, unfortunately, the treatment of rabbit manure lagged behind in the past compared to other animal manures. Currently, not only in China but also in Spain [
3], most rabbit manure is still treated through primitive and rough methods, that is, through natural air-drying. This method has many disadvantages, such as increasingly expensive land costs, serious pollution to the surrounding environment, and excessive dependence on the weather. In facing current problems and potential future applications, it is necessary to explore the value and application scenarios of rabbit manure.
Composting technology has been advocated by the government in the treatment of rabbit manure. Some existing studies have primarily verified the feasibility of composting rabbit manure (e.g., [
4]). Composting technology uses aerobic microorganisms to decompose the organic matter in the manure, followed by self-raising the temperature and killing harmful materials (e.g., roundworm eggs, pathogenic bacteria, and weed seeds) to achieve harmless products. At the same time, due to the formation of beneficial substances such as humus and plant hormones during the composting process, the agronomic effects could also be improved [
5]. Hence, composted manure has the potential to be used to manufacture organic fertilizer [
6].
Compost has also been explored to replace peat for manufacturing growing media to meet the great and potential needs of protected agriculture [
7] and internal green wall systems [
8]. Peat is an indispensable material for the manufacture of growing media, but the extraction of natural peat has been restricted in many countries due to its non-renewable characteristics [
9]. Therefore, using composted manure to replace peat would have good business and environmental prospects. Islas-Valdez et al. [
3] reported that the liquid biofertilizer obtained from the anaerobic digestion of rabbit manure could increase the grain yield of barley because of the essential nutrients and biostimulants. Similarly, good agronomic performance of rabbit manure compost was observed when it was used as organic fertilizer for okra [
10] and organic tomato [
11] or used as substrate for lettuce [
12]. In addition, the results from Cabanillas et al. [
13] indicate that vermicompost from rabbit manure was an effective alternative to urea in basil cultivation. Nevertheless, it is still unclear whether rabbit manure compost can be used to replace peat for growing media. This is because the requirements for use as fertilizer or growth media are different: the former is more concerned about nutrient effects (e.g., N, P, K-element contents), while the latter has special requirements for some physical and chemical properties (e.g., pH value, salinity, density, permeability, and water-holding capacity) [
14]. This has not been comprehensively reported in previous studies, and there are still knowledge gaps concerning the feasibility and agronomic effects of composted rabbit manures to replace peat as an organic matrix for manufacturing growing media.
In this content, the dynamic composting performance of three kinds of rabbit manure was monitored and the safety of the products (including heavy metal, germination index, and hygienic characteristics) was analyzed. Furthermore, the composts with the best performance were manufactured as growing media, and their physicochemical properties were evaluated, including pH value, electrical conductivity (EC), bulk density, and porosity. Then, the growing media were used for a seedling experiment in which the effects on the seedling emergence percentages and the growth status of cabbage seeds were evaluated. Finally, we provide some suggestions for further study and commercial applications of rabbit manure treatments.
2. Materials and Methods
2.1. Rabbit Manure
The rabbit manure was collected in a large-scale rabbit breeding herd in Jiyuan, Henan Province, China (35°08′ N, 112°57′ E). Three kinds of rabbit manure were compared and analyzed in this study to comprehensively evaluate their composting process. They were collected from a pregnant doe room (labeled as “R1”), an early-fattening rabbit (after weaning) room (labeled as “R2”), and a late-fattening rabbit room (labeled as “R3”), respectively. Urine was not included because the breeding herd used manure–urine separation technology.
2.2. Rabbit Manure Composting
2.2.1. Composting Preparation
The composting experiment was conducted in rabbit farms in November 2020. About 400 kg of fresh rabbit manure for each set of manure was piled into a conical stack with a triangular cross-sectional area. The initial moisture contents of the raw rabbit manures were ~60% (
Table 1) and thus suitable for composting, so that the moisture was not adjusted. Commercial composting additives (Organic fertilizer fermentation microbial agent, Shandong Lvlong Biological Technology Co., Weifang, China) were used to ensure successful composting, and 0.1 kg/ton fresh weight was added according to the product manual. The piles were manually turned over every two days, and the composting temperatures and ambient temperatures were measured every day. The composting test was completed until the composting temperatures were close to the ambient temperature, that is, 23 days into this study. In addition, some manures were air-dried and served as controls (without manually turning over), which were deposited on a 4–5 cm layer, and the drying test lasted one week and was completed after 8 days according to practice experience. The composting piles and air-dried layers were placed under a ventilated shelter, and the samples were collected after 1, 3, 6, 10, 14, 18, and 23 days, in turn, and were used for determination.
2.2.2. Analytical Methods
The fresh sample and deionized water were mixed at 1:10 (
v/
v) to obtain the sample extract, and its pH value was then measured using a pH meter (Sartorius PB-10, Sartorius AG, Göttingen, Germany); its EC was measured using a portable conductivity meter (Leici DDB-303A, Shanghai Oustor Industrial Corp., Shanghai, China). Moreover, the E4/E6 values and germination index (GI) were measured with reference to the reports of [
15] and [
16], respectively. Briefly, the E4/E6 values were obtained by calculating the ratios of the absorbance at 465 nm and 665 nm in the sample extract using an ultraviolet spectrometer (TU-1901 Ultraviolet-visible Spectrophotometer, Beijing Puxi General Instrument Co., Ltd., Beijing, China). GI were obtained by evaluating the germination of radish seeds (the number of germinated seeds and the length of the roots). The radish seeds were incubated using the sample extract at 25 °C in an incubator (CTHI -150B, STIK Instrument Equipment (Shanghai) Co., Ltd., Shanghai, China) in the dark for 48 h.
The moisture and organic matter content were determined by drying the sample at 105 °C for 24 h and calcining at 600 °C for 4 h, using an electric heating constant temperature blast drying oven (AL204, Mettler Toledo Instruments Ltd., Zurich, Switzerland) and a smart-box-type resistance furnace (SX2-8-10A, Shanghai Huyueming Scientific Instrument Co., Ltd., Shanghai, China), respectively; the total carbon content (TOC) was calculated according to the methods described in [
17]. In addition, the samples were dried and ground to pass through a 1 mm sieve to prepare the solid sample. Then, the solid sample was used to measure the total Kjeldahl nitrogen (TKN) via a modified semi-micro Kjeldahl procedure (KDY-9830, Beijing Tongrunyuan Electromechanical Technology Co., Ltd., Beijing, China) [
18]. The elements (including P, K, Cr, As, Cd, Hg, and Pb) were determined using inductively coupled plasma mass spectrometry (Agilent ICPMS7800, Agilent Technologies Co., Ltd., Palo Alto, CA, USA), and lignocellulose content (including cellulose, hemicellulose, and lignin) was determined using an automatic fiber analyzer (ANKOM A2000i, American ANKOM Co., Ltd., Macedon, NY, USA) according to the measures of [
19]. As regards hygiene, the mortality of roundworm eggs and the number of fecal coliforms were determined using the Chinese standard issues [
20,
21].
2.3. Growing Media Preparation
Only the composted R2 was used for the subsequent seedling experiment, due to its good nutritional value, hygiene, and GI compared to R1 and R3. The composted R2 was air-dried at room temperature for 7 days, and was then manually ground and sieved (5 mm). Then, the prepared compost was mixed with perlite (3–5 mm), vermiculite (2–4 mm), and peat in five different ratios to manufacture the growing media. For the five ratios, the composted manure was mixed at ratios of 0% (labeled as T0 treatment), 15% (labeled as T15 treatment), 30% (labeled as T30 treatment), 45% (labeled as T45 treatment), and 60% (labeled as T60 treatment), respectively. For each treatment, the ratios of both perlite and vermiculite were same, at 20%. The rest was mixed with peat. In other words, composted rabbit manure was used in different ratios (15–60%) to replace peat, which is the traditional material for growing media.
The pH and EC values were determined following the description provided in
Section 2.2.2. The bulk density, total porosity, and air space of the growing media were measured as described by [
22]. Briefly, a ring knife with a volume V was dried at 105 °C to a constant weight (recorded as
M1). Furthermore, the ring knife was filled with the sample and immersed in deionized water for 24 h and weighed (recorded as
M2). After the water was naturally removed by turning the sample upside down with sealing by gauze until no water dripped out, it was weighed (recorded as
M3). Finally, the ring knife was placed in an oven at 105 °C and weighed (recorded as
M4). The bulk density, total porosity, and air space were calculated using Equations (1)–(3), as follows:
2.4. Seedling Assay
The prepared growing media were used in the seedling experiment, which was conducted in a greenhouse at the China Agricultural University. Cabbage seeds (BEIJING XINSANHAO) were used in order to evaluate their agronomic effects; they were purchased from Jingyan Yinong Seed Sci-tech Co. (Beijing, China). For each treatment, seeding assays were repeated in triplicate at a polystyrene tray (72 cells), which means a total of 15 trays (5 treatment × 3 repetition) were conducted. These trays were randomly arranged in the greenhouse, where the temperature was controlled at 20 °C ± 5 °C and the humidity was about 60%. No additional fertilization was added.
At days 10, 15, and 20, the germination counts were performed and used for calculating the emergence rates in each growing medium. The experiment was completed on day 28. The seedling parameters (including the stem thickness, stem length, seedling height, fresh weight, and dry weight of the seedlings) were then determined. Briefly, five seedlings were randomly selected for each growing medium, and the stem diameter, seedling height (the distance from the growing medium surface to the top of the seedling), stem length, and root length were measured using a vernier caliper. The dry weight was determined by drying the samples at 105 °C for 15 min and at 80 °C after reaching constant mass. Chlorophyll (sum of chlorophyll a and chlorophyll b) was extracted and then measured following the method of [
23]. Briefly, 1 cm
2 of leaves was soaked in 5 mL of 80% acetone for 24 h; the optical density at 663 and 645 nm were then tested with a spectrophotometer (TU-1901 Ultraviolet-visible Spectrophotometer, Beijing Puxi General Instrument Co., Ltd., Beijing, China), and the total chlorophyll content was calculated using Equations (4)–(6), as follows:
In addition, the root/shoot ratio and the seedling vigor index were calculated using Equations (7) and (8), as follows:
Finally, the seedling quality of the growing media in each of treatments was comprehensively evaluated using the membership function [
24]:
where
f is the specific parameter in the growing media, including the stem diameter, stem length, root length, seedling height, aboveground fresh weight, belowground fresh weight, aboveground dry weight, belowground dry weight, and chlorophyll.
X is the measure value of the specific parameter;
Xmax and
Xmin are the maximum and minimum values of the parameter in all treatments.
All dates were the mean of three replicates, and the significant differences were analyzed through the Duncan test using SPSS 17.0. All figures were drawn using Origin 2021.