The identification of compounds with allelopathic potential produced by cover crops can assist in selecting species for weed management purposes in no-tillage systems. This study aimed to identify the main phenolic compounds with allelopathic potential in the shoot of rye (Secale cereale L.) and oilseed radish (Raphanus sativus L.) cover crops, as well as evaluate whether the cultivation system and phenological stage may influence secondary metabolite production and weed emergence. Samples of the shoot of these cover crops were collected at 60, 80, and 100 days after sowing (DAS) and 15 and 30 days after lodging (DAL) under field conditions. Weed emergence was evaluated at 45, 75, and 100 DAL of cover crops. The main compounds in rye were 6-methoxy-2-benzoxazolinone (MBOA) and 2-benzoxazolinone (BOA) under monocropping and intercropping, while flavonoid quercetin was found in oilseed radish at all evaluated times. During the growing cycle, the highest contents of phenolic compounds were found at the elongation stage (60 DAS) of rye under monocropping and intercropping systems (9.33 and 8.22 mg g-1 DM, respectively) and at grain filling stage (100 DAS) for oilseed radish intercropped with rye and black oat (3.24 and 3.83 mg g-1 DM, respectively). No differences were found in the contents of the main compounds when the species was grown under monocropping or intercropping systems. A reduction in the contents of MBOA, BOA, and quercetin was observed after lodging. Weed dry matter production was lower at 45 DAL in all treatments with rye and oilseed radish residues when compared to the control. The intercropping of rye with oilseed radish is an alternative management for weed control in agroecological systems due to the physical barrier created by these species and the presence of phenolic compounds with allelopathic potential.

Onion (Allium cepa L.) is a food crop of economic importance. In Brazil, the crop typically occurs in a conventional tillage system (CT), which favors the mineralization and decomposition of soil organic matter (SOM) and the loss of carbon (C) and nitrogen (N). On the other hand, the no-tillage vegetable system (NTVS) operates based on sustainable pillars and bypasses the adverse effects of CT. This study evaluated the total C and N stocks and particle-size fractions of SOM in NTVS with single and intercropped cover plants compared to vegetable crops under CT. The NTVS evaluated treatments were as follows: (1) spontaneous vegetation (SV); (2) black oats (BO); (3) rye (RY); (4) oilseed radish (OR); (5) RY + OR; and (6) BO + OR. A treatment under CT with millet cover, a no-tillage system with (NTS) millet + velvet + sunflower cover, and a forested area as the original condition was also evaluated. Soil samples were collected in 0–5, 5–10, and 10–30 cm layers. Stocks of total organic C (TOC), total N (TN), particulate OC (POC), particulate N (PN), mineral-associated OC (MAOC), and mineral-associated N (MN) were evaluated. The highest stocks of TOC, TN, POC, and NM were found in NTVS compared to CT, and RY + OR showed the best results. The NTVS showed higher TOC and TN stocks with grasses and cruciferous intercropped than NTVS with SV and CT. POC and PN stocks increased in areas with single and intercropped RY and OR treatments. MAOC and MN stocks were higher than forest in RY + OR intercrop in the topsoil layer. RY and OR intercrop efficiently added C and N to the soil under NTVS. The consortium of millet + velvet + sunflower in NTS showed higher TOC, TN, POC, and PN stocks compared to the other treatments (0–5 and 0–30 cm). In general, the intercrop of cover plants is ideal for obtaining the NTVS maximum potential, favoring several mechanisms between soil, plant, and atmosphere, resulting in improved soil quality, increased organic matter, and higher stocks of C and N.

Objective: The objective of this work was to evaluate the total carbon (C) and nitrogen (N) contents and the granulometric fractions of soil organic matter (SOM) under different management systems with single and intercropped winter cover crops in the cultivation of onion in Ituporanga, SC. Theoretical framework: Onion cultivation typically occurs in a conventional tillage system (CTC), which favors the loss of C and N from the soil due to soil mobilization. The vegetable no-tillage system (VNTS) uses sustainable practices with continuous soil cover and restricted soil tillage, promoting improvement of edaphic attributes. Methodology: The experiment was implemented in 2009 and the collection of soil samples took place in 2019 in the layers of 0-5, 5-10 and 10-30 cm in the treatments under VNTS, being black oat (AV); rye (CE); oilseed radish (NB); rye + radish (CE+NB); black oat + oilseed radish (AV+NB) and spontaneous vegetation (VE). In addition to these treatments, two adjacent areas were used, one under CTS and the other in a secondary forest area (MATA). The content of total organic C (TOC), total N (NT), light organic matter in water (LOM), particulate organic carbon (POC), particulate nitrogen (PN), C associated with minerals (COam) and N associated with to minerals (NAM). Results and conclusions: VNTS increased TOC and NT contents with the use of grass and cruciferous intercropping to the detriment of single crops and CTS. The LOM content was higher in VNTS with the use of single grasses and stood out in relation to VNTS with consortium. There were no differences for PN between MATA, CE and CE+NB, demonstrating that VNTS is efficient in raising these levels in substitution of CTS. The CE+NB consortium increased the levels of COam and NAM in the surface layer of the soil after replacing CTS with VNTS. Research implications: In VNTS, the use of intercropping between cover crops is an efficient management in adding C and N to the soil, as the simultaneous cultivation of different species of cover crops produces straw with an intermediate C/N ratio, the roots exploit different soil extracts and nutrient cycling is more efficient. Originality/Value: The results of this study are from a long-term experiment (10 years), which demonstrates the effectiveness of VNTS in recovering and increasing C and N levels in soil previously cultivated for a long period in the CTS, in addition to to match the forest area.

The presence of weeds in apple orchards affects the dynamics of nutrients in the soil, including phosphorus (P). The objective of this study was to evaluate changes in distribution of P fractions in the soil of an apple orchard under different weed managements. The experiment was conducted in an apple orchard in the municipality of Urubici, Santa Catarina, Brazil. The following treatments were implemented in 2011: no weed management (NWM), desiccation of weeds in the apple-tree row (DR), and hoeing of weeds in the apple-tree row (HR). Soil samples of the 0-2.5, 2.5-5, 5-10, 10-15 and 15-20 cm layers were collected in the apple-tree rows at 24 months after the implementation of the experiment. The samples were subjected to chemical fractionation of P, obtaining the following fractions: PiAER, PiNaHCO3, PoNaHCO3, PiNaOH, PoNaOH, PiHCl, PiNaOH05, PoNaOH05, and Presidual. The presence of weeds increased the contents of the following soil P fractions in the surface layers: PiAER, PiNaHCO3, and PoNaHCO3, which are bioavailable to plants. A higher proportion of organic forms of P in the soil was found when the weeds were hoed; these fractions can be mineralized and used for nutrition of apple trees when labile P forms are exhausted.

Most agricultural soils in North Carolina have high levels of available phosphorus (P), yet little is known about other accumulated P fractions and their associated environmental risks. This study characterized soil P fractions through sequential chemical fractionation of 391 samples (0–10 cm and 10–20 cm) collected from 57 counties across three regions (Tidewater, Coastal Plain, and Piedmont + Mountains). Metadata on soil management practices and crop history were also collected. Soil samples were chemically characterized and subjected to sequential fractionation using CaCl₂ (P _CaCl₂ ), Mehlich‐3 (P _M3 ), NaOH (Po _OH , Pi _OH ), and HCl (P _HCl ) extracts. Occluded P (P _OCL ) was determined by subtracting the sum of the previously extracted P fractions from the total P (P _T ), which was obtained through soil digestion. On average, P _OCL was the predominant fraction (39%), followed by Pi _OH (26%), P _M3 (17%), Po _OH (11%), P _HCl (6%), and P _CaCl₂ (0.3%). Organic P forms accounted for only 11% of the total extracted P. The highest P _M3 content was observed in Coastal Plain soils (79 ± 9 mg kg ⁻¹ ), which—like all regions—exceeded the critical soil test value for P in North Carolina (52 mg kg ⁻¹ ). Canonical discriminant analysis distinguished soil regions, with Piedmont + Mountains showing higher clay content and P _OCL , Coastal Plain linked to higher Fe, sand content, and P _M3 , and Tidewater linked to higher Al, acidity, humic matter, and C. Coastal Plain soils also exhibited the highest degree of P saturation (29%), followed by Piedmont + Mountains (24%) and Tidewater (15%), with 75% of the evaluated counties presenting a high risk of P contamination. Minimal differences in P fractions between tillage systems suggest excessive P fertilization and limited adoption of conservation practices, highlighting the urgent need for improved P management.