1. Introduction
Cultural practices that improve the sustainable production of regionally specific crops are needed to ensure the economic viability of rural areas and to protect natural resources. For example, sugarcane (
Saccharum spp. hybrids) is commercially produced on 340,000 ha exclusively in Louisiana, Florida, and Texas (USA), at a value of
$1.07 billion US annually [
1]. In Louisiana, the perennial crop is usually grown on a five-year cycle that includes an 8-month fallow period followed by at least 6-months of crop establishment with limited ground cover. However, soil left bare is highly erodible due to intensive tillage required to terminate the old ratoons and plant the new seed cane, as well as annual rainfall amounts exceeding 1650 mm [
2]. Historically, it was customary to rotate a leguminous crop alone, or intercropped with corn (
Zea mays L.), to serve as a green manure between sugarcane plantings [
3]. Today, many farmers in the US grow sugarcane as a monoculture, a practice that may limit profitable and sustainable productivity. Soil is degraded by the monoculture practice; it is more compacted [
4], contains less organic matter [
5], and abundant root pathogens that are harmful to crop health, including
Pachymetra chaunorhiza Croft and Dick [
6] and
Pythium arrhenomanes Drechs. [
7], proliferate. Degraded soils contribute to sugarcane yield decline observed in Australia [
8], Mauritius [
9], and Ethiopia [
10]. Stirling [
11] presented a four-tiered approach used in Australia to improve sugarcane sustainability; it included crop residue retention, minimum tillage, a leguminous rotation crop, and controlled, GPS-guided equipment traffic.
Cover cropping may be a solution to not only soil erosion and degradation, but to control weeds, improve field trafficability, and increase soil organic carbon (C) and nitrogen (N) status [
12]. Soil loss was reduced by 70% using cover crops in olive (
Olea europaea L.) groves, when compared to intensive tillage [
13]; Oat (
Avena sativa L.) and brassica and legume + oat mixtures suppressed weed biomass by 73–85%, when compared to the fallow treatment [
14]. Sunn hemp residue reportedly reduced smooth amaranth (
Amaranthus lividus L.) germination (40%), plant height (48%), and dry weight (80%) [
15]. A cereal rye (
Secale cereale L.) winter cover crop grown following corn and soybean (
Glycine max (L.) Merr.) increased soil organic matter, particulate organic matter, and potentially mineralizable N, when compared to a fallow treatment, by up to 15, 44, and 39%, respectively [
16]. Similarly, sunn hemp (
Crotalaria juncea L.) and crimson clover (
Trifolium incarnatum L.) increased soil total C and N content within a corn-fallow cropping system by 43 and 75%, respectively, over a three-year period [
17].
Cover crop adoption likely depends on the effects of the cover crop on succeeding sugarcane crops. Thawaro et al. [
18] investigated the effects of growing sweet sorghum (
Sorghum bicolor (L.) Moench), sunn hemp, rice (
Oryza sativa L.), or soybean on the growth and yield of sugarcane planted 2 weeks after cover crops were terminated and residues plowed into the soil. They reported that although stalks counts were similar between treatments, cane yield was higher where rice (43%) and sorghum (23%) were previously grown. Neither sunn hemp (−0.4%) nor soybean (5.5%) affected yields in that study. Others observed that growing legumes between sugarcane rotations over 6–12 months improved succeeding sugarcane yields by up to 21% [
19]. In Mauritius, growing hyacinth bean (
Lablab purpureus (L.) Sweet cv. Rongai) as a break crop, along with reduced N fertilizer and tillage, increased succeeding sugarcane plant cane yields [
9]. In Zimbabwe, soybean, grown as a break crop, improved sugarcane yields over fallowing [
20]. Previous research in Louisiana demonstrated that soybean, when grown as a green manure crop, did not affect succeeding sugarcane yields of cultivars ‘CP 70-321’ or ‘L 99-226’ [
21]. Similar results were obtained for soybean grown for grain, where neither cane or sugar yields for sugarcane cultivars ‘L97-128’ or ‘HoCP 96-540’ were affected [
22]. Some evidence suggests that cover crop residue may exhibit an allelopathic effect on a subsequent sugarcane crop. For example, a kenaf (
Hibiscus cannabinus L.) cover crop reduced stand counts (12%), cane yield (10%), and sugar yield (10%) of a sugarcane cultivar ‘HoCP 96-540’, planted 10 d after cover crop termination; on the other hand, cowpea (
Vigna unguiculata (L.) Walp.) did not [
23]. However, further field studies are needed to determine if leguminous cover crops act antagonistically toward subsequent sugarcane crops.
Therefore, the objective of the research was to determine the effects of growing leguminous cover crops, instead of fallowing soil, on subsequent sugarcane yields. Both cowpea and sunn hemp were chosen as cover crops because of high biomass production potential, N fixation capacity, and seed availability. Historical evidence, passed down from growers and agronomists, suggests that terminating cover crops too close to planting sugarcane can lower yields due to a composting effect of cover crop residue on the recently planted sugarcane seed cane. Therefore, a second objective was to determine the half-life of different cover crop residues in soil at a range of temperatures characteristic of temperate sugarcane production regions.
3. Results
For the Alma Plantation cowpea cover crop study, the cover crop and N fertilizer two-way interaction affected stalk sucrose content and sucrose yield in the plant cane sugarcane crop (
Table 2). The stalk sucrose concentration was greater where cowpea was grown when no N fertilizer was applied. On average the increase in stalk sucrose concentration was about 9% in the non-fertilized, cowpea treatment, when compared to the remaining treatments. In plots with no N fertilizer, sucrose yield (kg/ha) was greater where cowpea was grown, when compared to fallow, by about 5000 kg/ha. Both fertilized treatments produced similar cane yield, stalk sucrose, and sucrose yield.
In the first sunn hemp cover crop study, the cover crop and N rate two-way interaction affected plant cane yield and sucrose yield, but not stalk sucrose concentration. (
Table 3). No cover crop or N rate main effects were detected. Cane yield was greater where sunn hemp was grown and cane was fertilized with 45 kg N/ha, compared to the no N addition, by 21%. For sucrose yield, the sunn hemp + 45 kg N/ha treatment was greater than either the fallow + 90 kg N/ha or sunn hemp with 0 kg N/ha, by 20 and 27%, respectively (
Table 3). The remainder of the treatments produced similar sucrose on a hectare basis. In the first ratoon crop, cane yield was affected by N rate, but not cover crop or the two-way interaction (data not shown). Increasing amounts of N fertilizer corresponded to greater first ratoon cane yields, with 68.9, 79.2, and 84.3 t/ha, resulting from 0, 45, and 90 kg N/ha, respectively. Fallowing (78.1 t/ha) and growing sunn hemp (76.8 t/ha) as a cover produced similar cane yields. For the first ratoon crop in the first sunn hemp study, stalk sucrose concentration varied by cover crop (data not shown). Cane grown in fallowed plots exhibited a higher stalk sucrose concentration, when compared to the sunn hemp cover crop plots. However, sucrose yield in the first ratoon crop was only affected by N rate, where supplemental N (either 45 or 90 kg/ha) improved yield over the 0 N rate.
In the second sunn hemp study, plant cane yield was affected by the cover crop, but not N rate, nor the two-way interaction, and mean separation was performed across fertilizer rate (
Table 4). Sunn hemp treatment produced lower cane yields, when compared to fallow, by about 12%. Neither stalk sucrose concentration nor sucrose yield was affected by either cover crop, N fertilizer, or the two-way interaction for this field study. The first ratoon crop yields in the second sunn hemp study were not evaluated due to the lack of statistical differences observed for stalk sucrose and sucrose yield in the plant cane crop.
In the cowpea cover crop study at the Iberia Research Station, the cover crop treatment affected plant cane yield and sucrose yield, but not stalk sucrose concentration. The two-way interaction for each variable was not significant. Sugarcane yield for plots previously cropped to cowpea produced 10% more cane and 11% more sucrose, when compared to fallow plots (
Table 5). The overall stalk sucrose concentration was 120.9 g/kg. The nitrogen rate main effect also affected plant cane and sucrose yield, but not stalk sucrose concentration (
Table 5). When combined across cover crop treatments, application of either 45 or 90 kg N/ha, increased cane yield by 13 and 22%, respectively, when compared to the no N control, with means of 90.8, 98.1, and 80.6 t/ha, respectively (
Table 5). Similarly, nitrogen application of 45 or 90 kg/ha improved sucrose yield across cover crop treatments by 16 and 23%, respectively, when compared to the no N control, with means of 11,130, 11,820, and 9620 kg/ha.
Neither cover crop nor N application rate, or the two-way interaction, affected the first ratoon crop in the Iberia cowpea test (data not shown). Mean cane yield (95.3 t/ha), sucrose concentration (103.4 g/kg), and sucrose yield (9860 kg/ha) compared to the plant cane averages of 89.8 t/ha, 120.9 g/kg, and 10,850 kg/ha, respectively.
The crop residue used for the laboratory study varied in its TC and TN content, and therefore C:N ratio. As expected, the legume residue exhibited lower C:N ratios (18–33) when compared to the monocot crops (45–148) (
Table 6). The linear regression correlation coefficients were not consistent by temperature, or by crop residue. The poorer fit, compared to other correlations (e.g., r
2 > 0.80), can be attributed to fitting a single trendline to what appeared to be multiple (usually two) kinetic rates. However, when a sequential two pool kinetic model was used, calculated half-lives (DT
50) were very similar to a single pool kinetic model. This is possibly an indication that the initial rapid phase of decomposition represented a relatively small proportion of the total residue C decomposed over the course of the experiment. Thus, longer-term accuracy was not reasonably increased by using a more complex model. Incubation temperature resulted in basil soil respiration values of 0.42 (11 °C), 0.99 (25 °C), and 1.90 mg CO
2-C/kg soil day
−1 (32 °C) for the 166 day experiment (data not shown). Half-lives of crop residue at 11 °C were found to be > 157 days, further indicating slow soil microbial activity and processing at that temperature. At 25 °C, DT
50 were between 82–224% lower than those observed at 11 °C, and at 32 °C, DT
50 were between 11 higher and 203% lower than those observed at 25 °C (
Table 6). At 11 °C, cowpea and sorghum exhibited the lowest DT
50; at 25 °C, cowpea and soybean residue exhibited lowest DT
50; and at 32 °C, cowpea and sorghum exhibited the lowest DT
50.
4. Discussion
The cultivation of soybean in rotation with sugarcane for use as a hay and/or green manure was a standard practice in Louisiana nearly a century ago, partially to take advantage of the N fixed by legume [
3]. Data indicated sucrose yield of up to 7% higher by average, over nine sugarcane crops, by growing soybeans, which were incorporated as a green manure, compared to soybean that was removed for hay. However, soybean haying followed by adding 45 kg mineral N ha
−1 increased sucrose yield by 13%, compared to soybean hay removal, possibly indicating that the cane crop N needs were not completely met by the green manure alone. In the field studies reported here, cover crop effects varied, but on average, produced between −10% to 20% higher sucrose ha
−1, when compared to the fallow, highest N rate treatment (
Table 2,
Table 3,
Table 4 and
Table 5). The results reflect those reported where sugarcane cultivar ‘HoCP 96-540’ produced similar yields in either fallow, or fields where cowpea was incorporated into soil as a green manure 100 days after planting [
23]. In two plant cane and one ratoon crop, incorporated cowpea or soybean produced similar yields of sugarcane cultivar ‘CP 96-1252’ as observed in fallow soil; whereas sweet corn reduced cane and sugar yield compared to one or more of the other cropping systems, including fallow, in one plant cane and one ratoon crop [
31]. In Florida, USA, the effects of mill mud, fertilizer, and green manure on sugarcane yields was investigated in a sandy soil with a low cation exchance capacity (2.6 cmol
c/kg) [
32]. They reported that soybean grown as a green manure increased plant cane yields by >30%, and exhibited 12, 20, and 24% greater sugarcane leaf N, K, and manganese, respectively, when compared to either fallow or forage soybean systems. But, in pairwise comparisons, green manure alone did not improve sucrose yield (7%), compared to fallow and mineral fertilizer. Green manure with mill mud and/or fertilizer resulted in significant gains in sucrose by 50%. However, other reports indicated that mixed legume/grass break crops grown for 6–12 months increased yields of the subsequent cane crop by 16%, but traditional fallow did not [
19].
Incorporation of legumes as green manures altered soil N status in sugarcane fields in Nigeria [
33]. In two consecutive plant cane crops (1999, 2000), after four weeks, cowpea (20.3 kg N/ha, 24.2 kg N/ha), soybean (23.4 kg N/ha, 26.3 kg N/ha), sesbania (
Sesbania rostrata) (24.5 kg N/ha, 28.5 kg N/ha), and mucana (
Mucuna pruriens) (28.8 kg N/ha, 30.1 kg N/ha) incorporation increased soil N levels, compared to the no-fertilizer control (0.09 kg N/ha, 0.3 kg N/ha), but only to about 50% of 120 kg ha
−1 of mineral fertilizer N (41.0 kg N ha
−1, 46.0 kg N ha
−1). Respective cane yields in green-manured plots were greater than the no-fertilizer control, but similar to the mineral fertilized plots, indicating that N source was not as important as N availability. Moreover, soil N levels at eight weeks after green manure incorporation were the same as for the mineral fertilized fields (<10 kg/ha), indicating the importance of synchronizing legume N mineralization to sugarcane nutrient uptake. Brazilian researchers studied the effect of rotating legumes within the typical fallow period for sugarcane on two oxisols under long term continuous sugarcane production (>30 y) [
34]. They found no impact on soil chemical properties, including soil organic matter, pH, K, or P, but they did find an increase in soil aggregation, and a decrease in soil bulk density. In Louisiana, the mineral N fertilizer is applied after the first winter period, equivalent to about 8 months after an August cover crop termination date, making it more difficult to justify the benefits of green manuring to soil N status that directly affects sugarcane yields. For example, nitrate-N and ammonium-N levels in soil that was green-manured (7.8–8.7 t/ha) with soybean were statistically similar in fallow soil after 200 d, and the plant cane crop did not respond to green manure, in terms of cane or sucrose yield, with or without 134 kg N/ha mineral fertilizer [
21]. In the laboratory experiment, the DT
50 (25 °C) for sunn hemp (107 d), soybean (85 d) and cowpea (86 d) residue indicate that by 200 d, 20–30% of the cover crop residue would remain in soil. Given the relatively low C:N ratio of these cover crop residues (17.7–33.2), minimal N immobilization would be expected. Thus, it is possible a significant amount of mineralized N, originating from cover crop residues, is not plant available because of either leaching below the root zone as nitrate-N, volatilization as ammonia or nitrous oxide, or uptaken by opportunistic weedy plants.
There is ample work suggesting a soil biological component contributes to degraded soils under continual sugarcane. Less research describes the benefits of cover crops and green manure to long-term sugarcane soils. However, soil biological property changes resulting from break crops, pasturing, fallowing, or continual sugarcane production was reported for multiple locations in Australia [
35]. In general, pasturing increased microbial biomass C, free-living nematodes, mycorrhizal fungi, and substrate utilization diversity, with respect to the remaining cropping systems. But, a break of 12 months (observed in the Bundaberg location) reportedly was not long enough to affect any of the biological properties measured [
35]. However, in Florida, USA, microbial biomass C, but not N, was higher following cowpea or soybean cover crops, or a sweet corn crop, when compared to fallowed soil between sugarcane rotations [
31]. However, 90 days after a cane crop was planted, the viable microbial biomass (by phospholipid fatty acid analysis) was similar across each cropping system [
31]. Soil sampled during legume growth (between sugarcane cycles) may permit some observations into the effects of break crops on soil microbial ecology and nutrient transformations. Ammonium oxidizing bacteria and archaea, responsible for the first step of nitrification, were lower in soil cropped to soybean (−44%) and peanut (−24%), compared to fallowed soil [
36]. However, the levels of nitrite oxidizing bacteria were consistent between cropping system. This is important because without the ammonium oxidizers, organic N mineralized from soil organic matter, crop residues, or other sources (e.g., mill mud) is less likely to be converted into nitrite and nitrate and subsequently made unavailable by leaching.